EP2438414A1

EP2438414A1 - Residual lifetime indicator for industrial products

Info

Publication number: EP2438414A1
Application number: EP10736825A
Authority: EP
Inventors: Marco Maschietti; Marco Bianchini
Original assignee: MONTALBANO INDUSTRIA AGROALIMENTARE SpA
Current assignee: MONTALBANO INDUSTRIA AGROALIMENTARE SpA
Priority date: 2009-06-04
Filing date: 2010-05-27
Publication date: 2012-04-11
Also published as: WO2010140178A1; ITGE20090036A1

Abstract

Device indicating the residual shelf-life of perishable consumer products, comprising a material having a property which can be varied when the temperature is varied according to a given function, operating means which acts on said material so as to induce it to visualize the aforementioned property, and indicator means associated with the material/operating means assembly, said indicator means indicating the residual shelf-life in relation to an actual expiry date of the product, said material being a fluid with a variable viscosity depending on the temperature, which flows through a duct with a given cross-section, said operating means comprising a member able to apply a pressure differential to said fluid, the flow of said fluid being associated with said indicator means, characterized in that said operating member includes a pair of fluid systems, at least one of which comprises several phases in equilibrium with each other.

Description

RESIDUAL LIFETIME INDICATOR FOR INDUSTRIAL PRODUCTS

TEXT OF THE DESCRIPTION

The present invention relates to systems for evaluating the influence of variations in temperature on perishable products, and in particular it relates to a device indicating the residual shelf-life of consumer products, particularly food products.

Most industrial products, and particularly products of the agro- alimentary or pharmaceutical industry, have properties which deteriorate, until no longer present, within a given time period which is generally referred to as the "use by" or expiry date of the product. However, it must be said that generally, as often shown on the packaging of said products, this date refers to the fully intact product as stored under optimum conditions; nevertheless, it is not always possible for the consumer to establish whether the product, before being purchased, has been stored in the most appropriate manner.

Document US-A-55311380 discloses a device which is able both to detect the variation in temperature to which a product, in particular a frozen product, has been exposed, and to indicate the amount of said variation by suitable means. However, this device does not provide any evaluation as to the actual residual shelf-life of the product and, therefore, the detected data cannot be easily communicated to the consumer who usually does not have either the knowledge or the technical capacity to make an evaluation of this kind. WO-A-99/44021 describes a time/temperature indicator where a small strip of a given material is subject to a tensile load, for example that of a spring, and the given material is able to vary its response to the tensile force depending on the temperature. In this way, by suitably choosing the extensible material and the means for applying a tensile load to said material, it is possible to obtain a device which provides an indication as to the residual shelf-life of the product with which said device has been associated. However, this type of solution, from the point of view of its practical implementation, poses a number of problems associated mainly with the choice of the materials in question; firstly, calibration of the indicator may give rise to major difficulties. Moreover, most of these materials may be toxic or in any case have a harmful effect, so that they are unsuitable for a device intended to be used on pharmaceutical or food products. Furthermore, this device gives rise to major complications when attempting to obtain an irreversible indication as to the residual shelf-life of the product. Patent application WO-A-2006/128746, filed in the name of the same applicant, describes a device indicating the residual shelf-life of perishable consumer products, comprising a material having a property which can be varied when the temperature is varied according to a given function, operating means which acts on said material so as to induce it to visualize the aforementioned property, and indicator means associated with the material/operating means assembly, said indicator means indicating the residual shelf-life in relation to an expiry date of the product; said material being a fluid with a variable viscosity depending on the temperature, which flows through a duct with a given cross-section, the operating means comprising a member able to apply a substantially constant pressure to said fluid, the flow of said fluid being associated with said indicator means.

Italian patent application GE2008A000027, filed in the name of the same applicant, describes a device indicating the residual shelf-life of perishable consumer products, comprising a material having a property which can be varied when the temperature is varied according to a given function, operating means which acts on said material so as to induce it to visualize the aforementioned property, and indicator means associated with the material/operating means assembly, said indicator means indicating the residual shelf-life in relation to an actual expiry date of the product, said material being a fluid with a variable viscosity depending on the temperature,

2 substitute sheet Rule 26 which flows through a duct with a given cross-section, the operating means comprising a member able to apply a pressure differential to said fluid, the flow of said fluid being associated with said indicator means; a means for activating said member able to apply a pressure differential to said fluid is also provided, so as to distinguish between the manufacturing condition and the operating condition of said device.

In particular, said means able to control the generation of said pressure differential may comprise a removable barrier arranged between a chamber containing a more concentrated solution and a chamber containing a more diluted solution; indeed, the removal of said barrier creates the osmotic pair establishing the pressure differential applied to said viscous fluid.

In a further embodiment, said member able to apply a pressure differential to said fluid comprises a chamber containing a gaseous fluid at a given pressure.

This latter embodiment offers undoubted advantages from the applicational point of view, compared to the previously described embodiment which makes use of the osmotic system; however, both systems have the disadvantage that, in order to obtain a system which is able to operate for time intervals of a certain duration, they must be made with dimensions, in particular those of the chambers, which are fairly large, therefore making it difficult to produce label-like devices which can be also suitable for medium- and small-size consumer products.

Another problem associated with this type of device is that they are especially based, from the point of view of detecting the variations in temperature, on the variations in properties of the viscous fluid. Indeed, this feature limits the range of possible applications of the device.

Therefore, the object of the present invention is to provide a device of the type described above which may be made with extremely small dimensions while maintaining a wide functionality also during prolonged

3 substitute sheet Rule 26 periods of use. A further object of the invention is to provide a device which has a further improved sensitivity to variations in temperature.

Therefore, the object of the present invention is a device indicating the residual shelf-life of perishable consumer products, comprising a material having a property which can be varied when the temperature is varied according to a given function, operating means which acts on said material so as to induce it to visualize the aforementioned property, and indicator means associated with the material/operating means assembly, said indicator means indicating the residual shelf-life in relation to an actual expiry date of the product, said material being a fluid with a variable viscosity depending on the temperature, which flows through a duct with a given cross-section, said operating means comprising a member able to apply a pressure differential to said fluid, the flow of said fluid being associated with said indicator means, characterized in that said operating member includes a pair of fluid systems, at least one of which comprises several phases in equilibrium with each other. Particularly, said fluid systems are arranged at the ends of said duct with respective pressure values slightly different from each other.

In an embodiment, said fluid system with several phases in equilibrium comprises a pure volatile liquid in equilibrium with its vapour phase. Preferably, both the fluid systems comprise a pure volatile liquid in equilibrium with its vapour phase.

Alternatively, said fluid system with several phases in equilibrium may comprise a mixture of at least two volatile liquids in equilibrium with its vapour phase.

In another embodiment, said fluid system with several phases in equilibrium comprises a solution of a volatile liquid and a non-volatile solute at a given concentration, in equilibrium with its vapour phase.

Furthermore, said fluid system with several phases in equilibrium can further comprises a supercritical component, namely a component which is

4 substitute sheet Rule 26 used at a temperature greater than the critical temperature thereof, and particularly a gaseous component.

From a constructive point of view, said device includes: two chambers which have substantially the same volume and communicate to each other via a capillary duct. Particularly, said chambers and said capillary duct are formed integrally from a layer of mouldable material, said layer being covered with a layer of transparent film.

Other advantages and features of the device according to the present invention will be apparent from the following description of an embodiment thereof, which is provided by way of illustration, and not by way of limitation, with reference to the accompanying drawing, wherein:

Figure 1 is a plan view of an embodiment of the device according to the present invention.

Figure 1 shows an embodiment of the indicator according to the present invention, in which 1 denotes the base support on which said device is formed. The support 1 has formed therein two suitably shaped cavities 101 and 201 which are connected together by means of the capillary duct 301 ; the cavity 101 is divided into two chambers 111 and 121 communicating via the channel 131. Similarly, the cavity 201 is divided into two chambers 211 and 221 communicating via the channel 231. A graduated scale 411 is arranged on the wall 401 of the support 1 , in the proximity of the edge of the chamber 221. A layer 501 of transparent film is arranged over the wall 401 of the support 1.

The operating principle of the device according to the present invention will become apparent from the following. After filling the chamber

121 with the high-viscosity liquid, especially polyisobutene, preferably with a molecular weight in the range of 700-1000 Da, and in particular with a molecular weight of 920 Da, the two cavities 101 and 201 are evacuated and then the chambers 111 and 211 are partially filled with two pure liquids. These pure liquids are chosen depending on their vapour pressure

5 substitute sheet Rule 26 characteristics when the temperature is varied; examples of such liquids may be, for example, ethanol and propanol, which are respectively used for the chamber 111 upstream of the chamber into which the viscous liquid is introduced, and for the chamber 211 downstream of the capillary duct and the chamber 221 into which the viscous liquid will flow out.

The amount of liquid to be loaded into each chamber should be greater than a minimum value, at which or below which there would be a complete vaporization of the liquid loaded, and less than a maximum value, at which or above which the chamber would be completely filled with said liquid. Assuming that there is a range of pressures which are not high, namely that the equation of the state of the ideal gases correctly represents the volumetric behaviour of gases and vapours, the minimum number of moles to be loaded into the system should be equal to:

, _ Pn(T) - V₁ (1)

"I, rain _TyP

_ Ps₂(T) ^■ V_{2 (2)}

where ni,_min and n₂,_min represent the moles of components 1 and 2 which are loaded into the respective chambers, p_si(T) and p_S2(T) represent the vapour pressures of the liquids 1 and 2, respectively, R is the universal gas constant, and T is the temperature of the system, expressed in degrees Kelvin. In the above relations, the vapour pressure is shown as a function of the temperature; particularly, it is an increasing function of the temperature with a typically exponential progression. This means that the minimum number of moles to be loaded into the two chambers depends on the maximum temperature contemplated for a correct operation of the indicator. If the number of moles loaded into the system is less than the minimum value, the liquid loaded vaporizes completely and it is in equilibrium in the vapour phase at a pressure lower than its vapour pressure. The minimum number of moles

6 substitute sheet Rule 26 to be loaded into the system corresponds to the maximum value of the specific molar volume admissible for obtaining a two-phase system. This value is given by:

RT

^%v - 7^m ^<4>

On the other hand, if the number of moles loaded is equal to or exceeds the maximum value given by the relations:

nⁱ>^maχ - ΪΠ^~ TΛ ^{ ' n^{2>maX =} V V2^~L {(PS2I 7 J-Λ) ⁽⁶⁾

where V_IL and V_2L represent the specific molar volumes of the pure liquids 1 and 2, then the liquid loaded will fill the chambers entirely. In other words, it is required that, for each of the two components:

ViL < — < ViV (7)

where nj represents the moles loaded for the component being examined, and Vj represents the volume of the respective chamber. If the moles loaded into each chamber lie between the limit values defined by the above relations, then the liquids loaded into the two chambers vaporize partially, resulting in liquid-vapour equilibrium conditions inside each chamber. Consequently, the pressure p_si(T) will be established inside the cavity 101 , while the pressure p_S2(T) will be established inside the cavity 201. If the liquid 1 is regarded as being the more volatile liquid, namely P_si>P_s2, the pressure gradient which is generated by the two systems in a liquid-

7 substitute sheet Rule 26 vapour equilibrium and able to propel the high-viscosity liquid will therefore be equal to:

AP = p_sl(T) - _Ps2(T) ^(β)

5

Assuming, still in general terms, that the control of the fluid is performed exclusively by the capillary duct 301 , namely that it has a cross-section which is considerably smaller than that of the indicator channel, then the pressure gradient may be assumed, with a high degree of accuracy, as being equal to 10 the loss of head of the high-viscosity liquid at the ends of the single capillary duct. Therefore, applying Poiseuille's law for the laminar flow of a Newtonian fluid inside cylindrical ducts, the following relation may be defined:

15 where:

• μ(T) denotes the viscosity of the liquid flowing through the capillary duct, which is a function of the temperature;

• Q_v denotes the volumetric flow rate of the high-viscosity fluid flowing 20 through the capillary duct R₂; obviously, this volumetric flow rate is equal to the flow rate through the indicator channel.

The speed of advancing movement of the coloured viscous liquid through the indicator channel will therefore be equal to:

namely:

OvJ

8 substitute sheet Rule 26 The relation (11) represents the equation for dimensioning of the indicator in question, with reference to the example illustrated. It provides the speed of advancing movement of the coloured viscous liquid as a function of the design parameters of the device and the temperature. The temperature exerts an action in two ways:

• it produces major variations of viscosity in the viscous liquid: particularly when the temperature is increased, the viscosity of the liquid decreases significantly and therefore the indicator moves more rapidly.

• it influences vapour pressure values and: in particular, the driving thrust of the system may also increase considerably with the temperature, depending on the particular choice of the pair of liquids which are loaded.

Essentially, if in the indicators according to the patents of the above- mentioned applicant, shortly referred to as visco-osmotic indicator and visco- pneumatic indicator, the response characteristic depends essentially on the action of the temperature on the viscosity of the liquid flowing through the capillary duct, i.e. on the element which resists movement, then, in this case, a significant variation in the temperature may also be obtained from the thrust produced by the driving element. This is a distinctive feature of this indicator when compared to the visco-osmotic indicator or visco-pneumatic indicator. A major advantage of the indicator operated by systems in phase equilibrium is the possibility of forming the cavities 101 and 201 with smaller dimensions compared to the visco-osmotic and visco-pneumatic indicators. For example, in a closed visco-pneumatic indicator, the cavities 101 and 201 should be considerably larger than the indicator channel, so that there is a limited expansion of the gas upstream and a limited compression of the gas downstream. In fact, if this were not so, then the pressure gradient as initially applied would be reduced to the point of being zero and therefore causing the system to be stopped. On the other hand, where a pair of fluid systems in phase equilibrium are present, the pressure inside the two chambers is kept fixed (with a constant temperature) at the values of the respective vapour

9 substitute sheet Rule 26 pressures. The mechanism which makes this possible is explained in qualitative terms, for the example in question, as follows: The vapour contained inside the higher-pressure chamber, pushing the high-viscosity liquid, expands. This expansion of the vapour, instead of causing a reduction in pressure, causes the evaporation of a portion of the liquid in equilibrium therewith, such as to keep the pressure constant at the value ps1. Then, as the high-viscosity liquid advances, the liquid 1 evaporates, keeping the pressure constant inside the cavity 101 ; similarly, inside the cavity 201 , the vapour as compressed by the advancing movement of the viscous liquid condenses partially, keeping the pressure inside the cavity 201 constant. This mechanism, which distinguishes this type of indicator from the visco- pneumatic indicator, can operate for as long as two phases in equilibrium are present inside the two chambers, namely for as long as not all the liquid 1 has evaporated inside the cavity 101 and not all the vapour has condensed inside the cavity 201. The possibility of forming the cavities 101 and 201 with significantly reduced dimensions means that the device may be reduced in size, with obvious advantages in terms of packaging of the product.

A further aspect in connection with the above is the possibility of operation of the indicator with a smaller pressure gradient compared to the case of the visco-pneumatic indicator. In fact, one of the ways of limiting the volume of the chambers in a closed visco-pneumatic indicator is to increase the initial pressure difference at the ends of the high-viscosity liquid. This allows to maintain a pressure gradient other than zero even in the case of chambers which are not too large, since the (upstream) expansion and (downstream) compression phenomena, although they reduce the thrust applied to the viscous liquid, have available an increased pressure gradient to be "used up" before the device is stopped. However, in order for a device to operate for several days, the speed of advancing movement must be very low: in order to achieve this with a high pressure gradient, the capillary duct should have a very small diameter. Typically, capillary ducts with an internal

10 substitute sheet Rule 26 diameter of the order of 100 μm are necessary, these requiring precision- machining and being relatively costly.

To summarise: while a visco-pneumatic indicator has limitations in terms of the possibility for reduction of scale and/or the need for very small capillary ducts, an indicator operated by systems in phase equilibrium can obtain smaller scales and lower applied pressure gradients, such as to allow the formation of capillary ducts with a larger cross-section. The technology to be used for the manufacture of the capillary duct depends on the size of the internal diameter: the greater the diameter the more likely it will be possible to use less costly machining processes.

In the configuration shown by way of example in Figure 1 , the system is closed as a whole. This means that the pressure gradient applied to the viscous liquid is not influenced in any way by the value of the pressure of the external environment. This represents an advantage in that it means that the operating characteristics are not dependent on the external pressure, which is subject to variations along with the atmospheric conditions and altitude, and it prevents a possible tampering with the device. The sole parameter which influences operation of the indicator is - as must be the case - the external temperature. The possibility of obtaining a driving effect (the pressure gradient applied) which increases with an increase in the temperature allows an increase in the thermal sensitivity of the indicator, compared to the visco- osmotic indicator and visco-pneumatic indicator. In fact, the reduction in the resistance to movement (viscosity) following an increase in temperature, which is present in all viscosity indicators, is accompanied by an increase in the pressure gradient applied.

Maintaining the deactivated condition of the indicator and subsequent activation when needed may be achieved by keeping the indicator at a temperature which is sufficiently lower than the temperature of use indicated on the product to be monitored, i.e. at a value less than or about the same as

11 substitute sheet Rule 26 the flow point of the high-viscosity liquid. In this way, although the high- viscosity liquid is subject to a small applied pressure gradient, it is unable to flow and will remain in the position assumed during its loading. When the indicator is removed from the refrigerated environment and applied to the product to be monitored, it will start to function.

In an alternative configuration, at least one of the chambers 101 and 201 is filled with a liquid, for example ethanol, admixed with a given amount of gas. The gas used could be air derived from an incomplete evacuation of the chamber itself before being filled with the liquid, or an inert gas such as nitrogen. If the amount of liquid loaded into the chamber is between a certain minimum value and a certain maximum value, an equilibrium is formed between a gas phase and a liquid phase. A certain amount of a gaseous component, typically a very small amount, is dissolved in the liquid phase while the liquid vaporizes partially, being mixed with the gas (a gas/alcohol gaseous mixture, for example). In this case a so-called "hybrid" system is obtained, i.e. a combination of the visco-pneumatic indicator, in which only a gas phase is present inside the two chambers, and the indicator described above in the present patent, inside the chambers of which there is no gaseous component, but a liquid-vapour equilibrium is formed following the complete evacuation of the air and the loading with the liquids. Assuming that the chamber contains gas, when the contents of said chamber expand, the "hybrid" system develops a new phase equilibrium which is located at a pressure lower than the previous equilibrium. However, the drop in pressure which occurs may be, depending on the values of the design parameters, less prominent than in the case of the visco-pneumatic indicator, since the reduction in pressure which would have occurred in the absence of the liquid is partially balanced by a further partial evaporation of the liquid component. Similarly, if the chamber were to contain gas, the compression of said chamber may cause, depending on the design values, an increase in pressure which is less prominent than that which would be obtained in a

12 substitute sheet Rule 26 visco-pneumatic indicator. Also from the point of view of fluid dynamics, therefore, said system has the potential to provide a "hybrid" system combining the visco-pneumatic indicator and the indicator as completely evacuated and loaded only with liquids. In the case of a gaseous component which is insoluble in the liquid phase, low pressure (applicability of the equation of state of ideal gases) and incompressible condition of the liquid phase (namely, the density of the liquid phase depends only on the temperature, being independent of the pressure), the pressure which is formed inside the chamber containing the system in liquid-gas equilibrium may be calculated by the following relation:

p Vs ^• (V - ntotVL) + (1 ^~ z) ^■ UtOtRT (₁₂)

V - zn_to_tV_L

where p_s is the vapour pressure of the liquid, V is the volume of the chamber, n_to_t indicates the total moles (liquid + gas) inside the chamber, V_L is the specific molar volume of the liquid, and z is the overall molar fraction of the liquid. This relation allows to determine the design of an indicator containing gas/liquid mixtures in at least one of the two chambers.

While this type of indicator does not allow the pressure to remain perfectly constant inside the cavities 101 and 201 , it has the advantage that it allows a greater number of design parameters to be modified in order to adjust the internal pressure of the chambers (amount of gas loaded into the system, amount of liquid loaded, volume of the chambers). In the case of the indicator loaded with pure liquids (no gas present), the applied pressure gradient depends exclusively on the type of liquids used. Instead, by modifying the amount of gas present in the system within the permissible design limits, it is possible to adjust effectively the pressure gradient which is applied to the viscous liquid using always the same and sole liquid within the system.

13 substitute sheet Rule 26 In a further configuration of the indicator based on phase equilibrium conditions, the cavities 101 and 201 are completely evacuated and then filled with two liquid mixtures which are formed by the same pair of components, except that they have a different composition. If the amounts of liquid loaded are within a certain range, in this case also a liquid-vapour equilibrium, characterized by a different vapour value which allows movement of the viscous liquid, will be formed inside the two chambers. As the viscous liquid moves inside the indicator channel, the equilibrium pressure inside the cavity 101 tends to diminish, while the equilibrium pressure inside the cavity 201 tends to increase, unlike the case of the indicator where the two cavities are each filled with a pure liquid (indicator with perfectly isobaric chambers). However, in this case also, depending on the values of the design parameters, it is possible to provide an indicator in which the variation in pressure inside each chamber is smaller than that which would be obtained in a visco-pneumatic indicator. Moreover, the amount of pressure difference which propels the viscous liquid may be calibrated, not only by varying the pair of liquids, but also by modifying the composition of the liquid mixtures loaded, without having to change the component liquids.

By way of a further alternative to the systems described above, at least one of the two chambers may be loaded with a solution of a non-volatile solute which is dissolved in a solvent (for example: a salt dissolved in a volatile solvent). For example, the cavity 101 may be loaded with a pure solvent having a certain vapour pressure, while the cavity 201 is filled with a solution of a non-volatile solute (for example: a sugar, a salt) with a given concentration. By filling the two cavities with amounts of liquids within certain limits, an equilibrium between a vapour phase and a liquid phase is formed. The pressure inside the cavity 101 will remain constantly equal to the value of the vapour pressure of the solvent; the pressure inside the cavity 201 will be only slightly lower due to the ebullioscopic effect caused by the non- volatile solute which results in lowering of the equilibrium pressure. In

14 substitute sheet Rule 26 particular, said lowering of the equilibrium pressure (ΔP_et_>) may be estimated, according to an at least initial evaluation, by the relation:

AP_eb = ιs - p_s - x (13)

where v is the number of moles of dissociated species which are present in the solution per mole of dissolved solute, and x is the molar fraction of the solute. By way of example, with a molar concentration of a divalent salt equal to 0.05, an ebullioscopic reduction of the vapour pressure equal to 10% is obtained. As a result it follows that, with a solvent which has 500 mbar of vapour pressure, it is possible to obtain an ebullioscopic reduction of the order of 50 mbar. Consequently, if the cavity 101 is loaded with the pure solvent and the cavity 201 is loaded with the saline solution, a pressure gradient, propelling the viscous liquid, of about 50 mbar is obtained. As the indicator advances, the pressure inside the cavity 101 remains constant at the value of the vapour pressure of the solvent; as regards the cavity 201 , the reduction in volume causes a partial condensation of the vapour. By designing the system in such a way that the amount of liquid inside the cavity 201 is much greater than that of the vapour, the dilution effect caused by the partial condensation decreases only slightly the ebullioscopic effect, ensuring a pressure gradient for propelling the viscous liquid, which is approximately constant during the entire working life of the indicator.

This latter system also suggests an alternative mode for the activation system. The indicator may be kept in the deactivated state if solvent and solute are initially kept separate inside the cavity 201 (for example: the solute inside a capsule in contact with the solvent). In this state, a pressure equal to the vapour pressure of the solvent is applied at both ends of the viscous liquid. Following mixing of the solute with the solvent inside the cavity 101 (for example as a result of breakage of the capsule containing the solute), the

15 substitute sheet Rule 26 pressure inside the chamber is lowered due to the ebullioscopic effect. The viscous liquid starts to move.

16 substitute sheet Rule 26

Claims

1. Device indicating the residual shelf-life of perishable consumer products, comprising a material having a property which can be varied when the temperature is varied according to a given function, operating means which acts on said material so as to induce it to visualize the aforementioned property, and indicator means associated with the material/operating means assembly, said indicator means indicating the residual shelf-life in relation to an actual expiry date of the product, said material being a fluid with a variable viscosity depending on the temperature, which flows through a duct with a given cross-section, said operating means comprising a member able to apply a pressure differential to said fluid, the flow of said fluid being associated with said indicator means, characterized in that said member able to apply a pressure differential includes a pair of fluid systems which are arranged at the ends of said duct and at least one of which comprises several phases in equilibrium with each other.

2. Device according to Claim 1 , wherein said fluid systems are arranged at the ends of said duct with respective pressure values slightly different from each other.

3. Device according to Claim 1 or 2, wherein said fluid system with several phases in equilibrium comprises a pure volatile liquid in equilibrium with its vapour phase.

4. Device according to Claim 1 or 2, wherein said fluid system with several phases in equilibrium comprises a mixture of at least two volatile liquids in equilibrium with its vapour phase.

5. Device according to Claim 1 or 2, wherein said fluid system with several phases in equilibrium comprises a solution of a volatile liquid and a nonvolatile solute at a given concentration, in equilibrium with its vapour phase.

6. Device according to Claim 3, wherein both said fluid systems comprise a pure liquid in equilibrium with its vapour phase.

17 substitute sheet Rule 26

7. Device according to any one of the preceding Claims 3 to 5, wherein said fluid system with several phases in equilibrium further comprises a supercritical component.

8. Device according to Claim 7, wherein said supercritical component is gaseous.

9. Device according to any one of the preceding Claims 1 to 8, wherein said device includes: two chambers which have substantially the same volume and communicate to each other via a capillary duct.

10. Device according to Claim 9, wherein said chambers and said capillary duct are formed integrally from a layer of mouldable material, said layer being covered with a layer of transparent film.

18 substitute sheet Rule 26