EP1945516B1

EP1945516B1 - Anti-tampering beverage container made of plastic material such as pe, pet, pvc or the like.

Info

Publication number: EP1945516B1
Application number: EP06807679A
Authority: EP
Inventors: Guido Ribi
Original assignee: Ribi Leon Antoine
Current assignee: Ribi Leon Antoine
Priority date: 2005-11-11
Filing date: 2006-10-31
Publication date: 2010-12-22
Anticipated expiration: 2026-10-31
Also published as: US20080257857A1; ATE492476T1; ITSV20050037A1; DE602006019123D1; WO2007054449A1; EP1945516A1

Abstract

An anti-tampering beverage container ( 1 ) made of plastic material such as PE, PET, PVC or the like intended to contain liquids and composed of a thin casing stretched and provided with a watertight closure cap ( 3 ), wherein it is made such to be with a containment volume that changes depending on the pressure difference between inner pressure (Pr) and external pressure (Pa), particularly the atmospheric one and it has a reference (Lo) for measuring the liquid level, so that with pressure changing inside the container changes of the liquid level are indicated due to the change of the containment volume so tampering actions being shown causing the container to be opened and so causing the inner pressure to change with respect to the original one under sealed condition.

Description

The present invention relates to an anti-tampering beverage container made of plastic material such as PE, PET, PVC or the like intended to contain liquids and composed of a thin casing stretched and provided with a watertight closure cap.
Due to various reasons all containers for food liquids can be subjected to a tampering action before they are opened to be used which tampering action can be intentional or not. Therefore such possible tampering action both intentional and caused by incorrectly handling the container is desired to be clearly noticed even by a little observing user.
The document US 2003/0173327 discloses a beverage container according to the preamble of claim 1, capable of being filled with hot liquid, in which the side-wall has an upper portion, a central portion a lower portion and a base. The central portion includes a vacuum panel portion having an initiator portion capable of flexing inwardly under low vacuum force and a control portion to invert and flex further inwardly into the container. In an embodiment a telescopic vacuum panel is capable of flexing inwardly under low vacuum force and enables expansion from the collapsed state when the container is uncapped and the vacuum released. Without the low vacuum force pulling the vertically folding vacuum panel section down, it will reserve in direction immediately due to the forces generated by the memory in the plastic material. This provides for a "tamper-evident" feature for the consumer, allowing as it does for visual confirmation that the prodiuct has not been opened previously.
Most of tampering-evident systems are at the level of the closure cap and generally they highlight only an opening due to the breaking of a part of the cap.
However, such devices, sometimes are not effective not only because they allow an opening without breaking due to their elasticity, but also because they do not show the tampering action caused to the container. These breakings of the container body can be both microcracks caused by inappropriate handlings or by the change of the material structure and also intentional tampering actions by piercing the container wall, often for introducing harmful substances in the container 1.
So the object of the present invention is to provide various structural shapes of containers allowing to clearly view a possible tampering action.
As it is known, liquids contained in containers made of plastic material, are generally kept under pressure mainly in order to give the container a certain rigidity, which pressure is obtained in various manners such as by the presence of dissolved CO₂, presence of N₂ dissolved by liquid N₂, generally compressed gases or by the necking down of full containers by heat changing the structure of the thermoplastic material.
The basic principle of the present invention is the fact of highlighting a change in the level of the contained liquid following a programmed change of the inner volume of the container, due to a change in pressure.
The concept is also valid for an inner pressure lower than the athmospheric pressure p_a that can be obtained both by decreasing the temperature of liquids and by condensing residual vapours.
Under the effect of these changes of pressure per the inner volume of the container increases or decreases and such change can be noticed by a relative level change, which level is indicated by the liquid meniscus in the neck of the container.
When this level l₀ in the full container and under a certain pressure p_r is intentionally brought to coincide with an irremovable mark on the neck of the container by a controlled filling, as a consequence of a change of the inner pressure, such as for example a tampering action, i.e. when the inner pressure p_r is equal to the atmospheric one p_a, the inner volume of the container changes and the level l₀ of the contained liquid too that will be observed with respect to the mark on the neck of the container.
Therefore the invention relates to an anti-tampering beverage container made of plastic material such as PE, PET, PVC or the like intended to contain liquids and composed of a thin casing stretched and provided with a watertight closure cap according to what described hereinbefore, which container is made such to be with a containment volume that changes depending on the pressure difference between inner pressure and external pressure, particularly the atmospheric one and it has a reference for measuring the liquid level, so that with pressure changing inside the container changes of the liquid level are indicated due to the change of the containment volume so tampering actions being shown causing the container to be opened and so causing the inner pressure to change with respect to the original one under sealed condition.
A preferred embodiment provides the container wall to be elastically extendable completely or for a part thereof in order to produce changes in the containment volume depending on the pressure difference between inner pressure and the one outside the container.
A particular embodiment of the invention provides an anti-tampering beverage container made of plastic material such as PE, PET, PVC or the like intended to contain liquids characterized in that it is composed of a thin casing stretched and provided with a watertight closure cap in whose side part, i.e. in the skirt, there are provided one or more annular folds like a general bellows whose axial sections, whose shapes being any appropriate ones, on one hand have a resting steady condition, i.e. corresponding to the lack of inner elastic stresses of their plastic material, and on the other hand have inner elastic stress conditions of their plastic material when are stressed by axial forces generated by differences of inner pressure p_r of the container with respect to the atmospheric pressure p_a such that when the inner pressure p_r of the container increases with respect to the atmospheric one p_a, said folds extend increasing its volume, whereas, on the contrary, when the inner pressure p_r decreases with respect to the atmospheric one p_a, said folds compress decreasing the inner volume of the container so then, in both above cases, they return back to initial volume resting conditions when the atmospheric pressure p_a is restored inside the container and so, as a consequence of the return of said initial resting volume conditions, specifically caused by a possible tampering action, an increase of level d2 relevant to the contained liquid in the case of a decrease in the inner pressure p_r of the container, and a decrease of the level relevant to the contained liquid in the case of an increase of inner pressure p_r of the container respectively will be noticed by appopriate marks placed at the neck region of the container.
Still according to a further characteristic planes of sections of the container defined by bellows-like folds are perpendicular to the longitudinal axis of the container.
A further embodiment of the invention provides an anti-tampering beverage container made of plastic material (PE, PET, PVC or the like) intended to contain liquids and composed of a thin casing stretched and provided with a watertight closure cap, whose bottom has such a shape that it elastically gets deformed, bending towards the outside, causing the inner volume of the container to increase if its inner pressure p_r increases and it elastically gets deformed bending towards the inside causing the inner volume of the container to decrease if its inner pressure decreases p_r<p_a, while such volume changes are highlighted by suitable level marks placed preferably at the neck region of the container at the filling level (L0).
Still a different embodiment provides an anti-tampering beverage container made of plastic material such as PE, PET, PVC or the like intended to contain liquids, characterized in that it is composed of a thin casing stretched and provided with a watertight closure cap, whose side surface, i.e. its skirt, is made such that at least a part of its sections, perpendicular to the longitudinal axis have a star-shape configuration, i.e. they have longitudinal bellows-like folds that are straight or also like spirals allowing the inner volume to increase with the inner pressure of container increasing p_r>p_a and allowing the inner volume to decrease with the inner pressure decreasing p_r<p_a, whereas such volume changes are highlighted by suitable level marks suitably placed at neck regions of the container.
According to a variant embodiment an anti-tampering beverage container made of plastic material such as PE, PET, PVC or the like intended to contain liquids according to the present invention is composed of a thin casing stretched and provided with a watertight closure cap and it has a specific mark placed at the level of the container neck, and wherein said casing is made by different manners such as by adding suitable additives, by means of moulding temperatures and/or stretching rates in the mould programmed for obtaining a material having a modulus of elasticity E, enough low to highlight a change of the inner volume of the container when the inner pressure p_r changes by means of said mark.
A further embodiment of the invention provides also an anti-tampering beverage container made of plastic material such as PE, PET, PVC or the like intended to contain liquids and composed of a thin casing stretched and provided with a watertight closure cap and provided with a suitable mark placed at the container neck and wherein said casing has sections of its side walls that are perpendicular to the longitudnal axis with different diameters along the axis, or also sections perpendicular to the longitudinal axis having a ratio (border section development/section surface) greater than 4/Ø, where Ø = diameter of the circle whose circumference is equal to the development of the border of the considered section such to highlight a change in the inner volume of the container when the inner pressure changes by means of said mark.
Further characteristics are the object of subclaims.
These and other characteristics and advantages of the present invention will be more clear from the following description of some embodiments shown in annexed drawings wherein:

Fig. 1 is a container 1 with bellow-like folds 6 perpendicular to the longitudinal axis (a÷a'), in two configurations a) p_r>p_a and b) p_r=p_a.
Fig.2 is a container 1 with a corrugated, elastic bottom 5 in two configurations: a) p_r=p_a, b) p_r<p_a and c) p_r>p_a and wherein sections s1 and s2 can be noted containing ridges 7 cs and ci.
Fig.3 is a container 1 with cup-like elastic bottom 5 with vertex towards the inside, in two configurations: a) p_r=p_a and b) p_r>p_a;
Fig.4 is a container 1 with a cup-like elastic bottom 5 with the vertex towards the outside in two configurations: a) p_r=p_a and b) p_r<p_a;
Fig.5 is a container 1 with a star-like section 8 normal to the longitudinal axis (a÷a') :
1. a) (b1÷b1') section with p_r=p_a and
2. b) (b2÷b2') section with p_r>p_a
3. c) general star-like section 8
Fig.6 is a container 1 with a low value of the modulus of elasticity E, in two configurations a) p_r=p_a and b) p_r>p_a;
Fig.7 is a container 1 with not circular sections:
1. a) longitudinal section with p_r=p_a
2. b) orthogonal section with p_r=p_a
3. c) orthogonal section with p_r>p_a;
Fig.8 is a container 1 with different section diameters in the skirt, where h = h (p_r).

As it is known, liquids contained in containers 1 made of plastic materials, are generally kept under pressure mainly for giving the container 1 a certain rigidity, which pressure is obtained in various ways such as by the presence of dissolved CO₂, presence of N₂ dissolved by liquid N₂, generally compressed gases or by the necking down of full containers 1 by heat changing the structure of the thermoplastic material.
The basic principle of the present invention is the fact of highlighting a change in the level of the contained liquid following a programmed change of the inner volume of the container 1, due to a change in pressure.
The concept is also valid for an inner pressure lower than the athmospheric pressure p_a that can be obtained both by decreasing the temperature of liquids and by condensing residual vapours.
Under the effect of these changes of pressure p_r the inner volume of the container 1 increases or decreases and such change can be noticed by a relative level change, which level is indicated by the liquid meniscus in the neck of the container 1.
When this level l₀ in the full container 1 and under a certain pressure p_r is intentionally brought to coincide with an irremovable mark 4 on the neck of the container 1 by a controlled filling, as a consequence of a change of the inner pressure, such as for example a tampering action, i.e. when the inner pressure p_r is equal to the atmospheric one p_a, the inner volume of the container 1 changes and the level l₀ of the contained liquid too that will be observed with respect to the mark 4 on the neck of the container 1.
All these general aspects concern all embodiments of the present invention. Therefore each one of these embodiments allows a change of the inner volume of the container 1 following a change of pressure p_r that can be noticed by a corresponding change of the level L₀.
In a first embodiment (fig.1) a and b, the container 1 made of plastic material is molded from a standard preform inside a special mould at the negative side having one or more annular bellows-like folds 6 having an appropriate depth and angularity.
Folds 6 on the container 1, obtained from the mould, have a their own basic structural position and are not subjected to any external forces.
With the container 1 full and closed, a change of pressure p_r due to any reasons, will lead to an elastic and reversible stretching of folds 6 and therefore the inner volume will increase if pressure increases p_r>p_a and to an elastic and reversible compression of folds 6 and so the inner volume will decrease if pressure decreases p_r<p_a.
The increase in pressure p_r>p_a can be obtained by one of the usual methods already mentioned, whereas a decrease in pressure p_r<p_a can be preferably obtained by filling the container 1 with its contents that has been previously heated paying attention not to overcome the lowest temperature of the vitreous passage of the material followed by a cooling, even a natural one, that will lead to a condensation of residual vapours and to a decrease in partial pressures. The mark 4, obtained directly in the mould or subsequently applied, will be brought to coincide with the provided filling level L₀. Therefore it is obvious that said folds 6 have to be all under the lowest provided level.
In a second embodiment (fig. 2), the container 1 has a configuration of the bottom 5 that can be deformed. This configuration is composed of a corrugated surface with concentric circles having waves with a suitable shape, in this case the mould will be more complicated. However surfaces tangent to ridges 7 both to the upper ones c_s and to the lower ones c_i will be placed above the supporting plane of the container 1. Such corrugated surface must have the possibility to get deformed along the longitudinal axis (a÷a') of the container 1 when the pressure changes, i.e. it will bend towards the inside if the pressure decreases p_r<p_a and it will bend towards the outside if the pressure increases p_r>p_a.
The corrugated configuration will allow considerable elastic reversible deformations with minimum radial stresses. Obviously there will be a balancing position with an inner pressure p_r=p_a equal to the atmospheric one.
The fact of bending towards the outside will have to be such not to lead the deformation beyond the supporting plane (p÷p'). It is obvious that such deformations will lead to changes in the inner volume with modes similar to the ones provided in the first embodiment as well as considerations and characteristics concerning the mark 4 at level l₀ will be valid.
In a third embodiment (fig. 3) the bottom 5 in its central part is composed of a cup-like suface revolving about the longitudinal axis (a÷a') of the container 1 with the vertex faced towards the inside of the container 1. This shape is provided for deformations caused by an increase in pressure p_r>p_a. Under the effect of the inner pressure p_r said central part or cup will bend elastically and reversibly towards the outside causing the inner volume to increase: therefore it can return to its initial configuration with the initial pressure coming back p_r=p_a. Considerations concerning the change of volume and level, following the increase in pressure p_r>p_a as well as the ones concerning the mark 4 of the container 1 obtained in the mould too, or subsequently applied thereto, are the same as the ones mentioned for the first and second embodiment.
In a fourth embodiment (fig.4) the bottom 5 of the container 1 made of plastic material, in its central part is composed of a cup-like surface revolving about the longitudinal axis (a÷a') with the vertex faced towards the outside of the container 1. This shape is provided for deformations caused by a decrease in pressure p_r<p_a. Under the effect of the inner depression, said central part bends elastically and reversibly towards the inside causing the inner volume to decrease: therefore it can return to its initial configuration with the initial pressure coming back p_r=p_a. As in previous embodiments, considerations concerning changes in volume with the pressure changing as well as the ones concerning the mark 4 that is an integral part of the container 1, are always the same.
In a fifth embodiment (fig.5) the container 1 made of plastic material such as PE, PET, PVC or the like, is still made by a mould starting from a standard preform. In this case the container 1 has bellow-like folds 6 along the skirt, i.e. said folds 6 are parallel to the longitudinal axis (a÷a') of the container. Said folds 6 can be also arranged along spirals rotating about the longitudinal axis. The section of the skirt normal to the longitudinal axis (a÷a') therefore will have in both cases a corrugated or star-like shape 8 with a variable number of vertices. However in this embodiment there are problems about the mould that necessarily has to be composed of many pieces and also about time for manufacturing containers 1. In this embodiment, except for the arrangement of bellows-like folds 6, nothing is changed as regards operation with respect to previous embodiments.
In a sixth embodiment of the anti-tampering container 1 made of plastic material such as PE, PET, PVC or the like (fig.6) the problem of expanding the inner volume is not more solved by using shapes or structures aiding the change of volume such as in above mentioned embodiments. The container 1 is made of a plastic polymer with a low modulus of elasticity. This polymer can take desired mechanical characteristics both for the presence of suitable additives and for different modes stretching the preform in the mould, the main influencing parameters being temeperature and stretching rate. As in all previous embodiments, the increase in volume and its subsequent decrease following a possible tampering action are noticed by means of the mark 4 provided at the filling level L₀.
Other embodiments of the anti-tampering container 1 made of plastic material can be made in various configurations provided that they satisfy the need of a considerable change in volume and so in the contained liquid depending on a small change in pressure. In all these embodiments said change of the level can be noticed by means of the mark 4 placed at the provided filling level.

For these other embodiments, two structure assemblies that are conceptually different one from the other are claimed in claim n. 15; the first one concerns different diameters of the skirt along the longitudinal axis (a÷a') mainly allowing axial changes of volume and the second one concerns skirt sections normal to the longitudinal axis (a-a'), that are different with respect to the circular ones, prevalently allowing radial changes in volume.

LEGEND OF REFERENCE NUMBERS

1)	Container
2)	Container neck
3)	Cap
4)	Level mark
5)	Bottom
6)	Bellows folds
7)	Ridges of the corrugated bottom
	c_s upper ridges
	c_i lower ridges
8)	star-like section
(a÷a')	= longitudinal axis
(b÷b')	= sections of vertical bellows-like container
(p÷p')	= supporting plane of the container
L₀)	= filling level
l₀)	= meniscus level
d₂)	= level difference
p_r)	=inner pressure of the container
p_a)	=atmospheric presure
Ø)	=diameters
(π÷π') =	planes of folds (6).

Claims

An anti-tampering beverage container (1) made of plastic material such as PE, PET, PVC or the like intended to contain liquids and composed of a thin casing stretched and provided with a watertight closure cap (3), characterized in that it is made such to be with a containment volume that changes depending on the pressure difference between inner pressure and external pressure, particularly the atmospheric one, and it has a reference for measuring the liquid level, so that with pressure changing inside the container (1) changes of the liquid level are indicated due to the change of the containment volume therefore indicating tampering actions caused by opening the container (1) and so causing the inner pressure to change with respect to the original one under sealed condition.
Container (1) according to claim 1, characterized in that the container (1) wall is elastically expandible completely or for a part thereof in order to produce changes in the containment volume depending on the pressure difference between inner pressure and the one outside the container (1).
Anti-tampering beverage container (1) made of plastic material such as PE, PET, PVC or the like intended to contain liquids according to claim 1 or 2, characterized in that it is composed of a thin casing stretched and provided with a watertight closure cap (3) in whose side part, i.e: in the skirt, there are provided one or more annular folds (6) like a general bellows whose axial sections, whose shapes being any appropriate ones, on one hand have a resting steady condition, i.e. corresponding to the lack of inner elastic stresses of their plastic material, and on the other hand have inner elastic stress conditions of their plastic material when are stressed by axial forces generated by difference of inner pressure (p_r) of the container (1) with respect to the atmospheric pressure (p_a) such that when the inner pressure (p_r) of the container (1) increases with respect to the atmospheric one (p_a), said folds (6) extend increasing its volume (1), whereas, on the contrary, when the inner pressure (p_r) decreases with respect to the atmospheric one (p_a), said folds (6) compress decreasing the inner volume of the container (1) so then, in both cases, they return back to initial volume resting conditions when the atmospheric pressure (p_a) is restored inside the container (1) and so, as a consequence of the return of said initial resting volume conditions, specifically caused by a possible tampering action, an increase of level (δ2) relevant to the contained liquid in the case of a decrease in the inner pressure (p_r) of the container (1), and a decrease of the level relevant to the contained liquid in the case of an increase of inner pressure pr of the container (1) respectively will be noticed by means of appropiate marks (4) placed at the neck (2) region of the container (1).
Anti-tampering beverage container (1) according to one or more of the preceding claims, characterized in that planes (π-π') of sections of the container (1) defined by bellows-like folds (6) are perpendicular to the longitudinal axis (a÷a') of the container (1).
Anti-tampering beverage container (1) according to one or more of the preceding claims, characterized in that it comprises means for viewing a change of the level of the liquid contained therein composed of marks (4) or indications obtained in the material of the container (1), placed at the neck region (2) of the container (1), in the form of preferably annular reliefs (4), recesses or grooves or indentations that define at least a plane perpendicular to the longitudinal axis at the filling level (L₀).
Anti-tampering beverage container (1) according to one or more of the preceding claims, characterized in that marks (4) or indications, about which at claim n. 5, are replaced by tapes or also cords irremovably glued or sealed to the plastic material of the container (1).
Anti-tampering container (1), made of plastic material (PE, PET, PVC or the like) intended to contain liquids, according to claims 1, 2, 5 or 6 characterized in that it is composed of a thin casing stretched and provided with a watertight closure cap (3), whose bottom (5) has such a shape that it elastically gets deformed, bending towards the outside, causing the inner volume of the container (1) to increase if its inner pressure (p_r) increases and it elastically gets deformed bending towards the inside causing the inner volume of the container (1) to decrease if its inner pressure decreases (p_r<p_a), whereas such volume changes are highlighted by suitable level marks (4) placed preferably at the neck region (2) of the container (1), such one of the ones claimed at claim 5, at the filling level (L₀).
Anti-tampering beverage container (1) according to claim 7, characterized in that it has a bottom (5) composed of concentric waves (7) obtained in the material whose upper ridges (c_s) and lower ridges (c_i) and circles containing all intermediate homologous points between said ridges (7) (c_s and c_i) are all concentrically arranged about the longitudinal axis (a÷a') of the container (1) and are contained in planes perpendicular to this axis (a÷a') whereas, the two surfaces containing ridges (7) the upper ones (c_s) and the lower ones (c_i) are both placed above the supporting plane (p÷p') of the container (1).
Anti-tampering beverage container (1) according to claim 7, characterized in that an increase of inner pressure (p_r>p_a) of the container (1) causes the bottom (5) to elastically and reversibly bend towards the outside thus increasing the inner volume of the container (1) and a subsequent return to initial pressure (p_a) allows the bottom (5) to restore its initial configuration.
Anti-tampering beverage container (10) according to claim 7 characterized in that a decrease of the inner pressure (p_r<p_a) of the container (1) causes the bottom (5) to elastically and reversibly bend towards the inside thus decreasing the inner volume of the container (1) whereas a subsequent return to initial pressure (p_a) allows the bottom to restore its initial configuration.
Anti-tampering beverage container (1) according to claims 7 and 9, characterized in that it has a bottom (5) in which at least a central part of its surface is a surface revolving about the longitudinal axis (a÷a') of the container (1) with the vertex faced towards the inside of the container (1) and whose radii of curvature and depths of plastic material are such that they allow said central to reversibly elastically bend towards the outside when the inside part of the container (1) is subjected to an increase of pressure (p_r>p_a) and subsequently allow the initial configuration to be restored with the initial pressure (p_a) coming back.
Anti-tampering beverage container (1) according to claims 7 and 10, characterized in that it has a bottom (5) in which at least a central part of its surface is a surface revolving about the longitudinal axis (a÷a') of the container (1) and whose radii of curvature and depths of the plastic material are such that they allow a said central part to reversibly elastically bend towards the inside when the inside part of the container (1) is subjected to a decrease in pressure (p_r<p_a) and subsequently allow the initial configuration to be restored with the initial pressure (p_a) coming back.
Anti-tampering beverage container (1) made of plastic material such as PE, PET, PVC or the like intended to contain liquids, according to claims 1, 2, 5 or 6 characterized in that it is composed of a thin casing stretched and provided with a watertight closure cap (3) characterized in that its side surface, i.e. its skirt, is made such that at least a part of its sections perpendicular to the longitudinal axis (a÷a') have a star-like configuration (8), that is have longitudinal bellows-like folds (6) that are straight or also like spirals allowing the inner volume to increase with the inner pressure of container (1) when increases (p_r>p_a) and allowing the inner volume to decrease with the inner pressure decreases (p_r<p_a), whereas such volume changes are highlighted by suitable level marks (4) suitably placed at neck regions (2) of the container (1), such as one of the ones claimed at claims 5 and 6.
Anti-tampering beverage container (1) made of plastic material such as PE, PET, PVC or the like intended to contain liquids according to claims 1, 2, 5 or 6 characterized in that it is composed of a thin casing stretched and provided with a watertight closure cap (3) and in that it is provided with a specific mark (4) placed at the level of the neck (2) of the container (1), such as one of the ones claimed at claims 5 and 6, and wherein said casing is made by different manners such as by adding suitable additives, by means of moulding temperatures and/or stretching rates in the mould programmed for obtaining a material having a modulus of elasticity E, enough low to highlight a change of the inner volume of the container (1) when the inner pressure (p_r) changes by means of said mark (4).
Anti-tampering beverage container (1) made of plastic material such as PE, PET, PVC or the like intended to contain liquids according to claims 1, 2, 5 or 6 characterized in that it is composed of a thin casing stretched and provided with a watertight closure cap (3) and in that it is provided with a suitable mark (4) placed at the neck (2) of the container (1), such as one of the ones claimed at claims 5 and 6, and wherein said casing has sections of its side walls that are perpendicular to the longitudnal axis (a÷a') with different diameters along the axis (a÷a'), or also sections perpendicular to the longitudinal axis (a÷a') having a ratio (border section development/section surface) greater than 4/Ø, where Ø = diameter of the circle whose circumference is equal to the development of the border of the considered section such to highlight a change in the inner volume of the container (1) when the inner pressure (p_r) changes by means of said mark (4).