GB2168147A - Temperature change indicating device - Google Patents

Abstract

A device for indicating a rise in temperature above a predetermined value comprising a quantity of a first material (5) which changes from a solid state to a liquid stage and consequently causes a subsequently detectable change in the device when its temperature rises above said predetermined value; and a quantity of a second material (7) which, when said first material (5) becomes liquid, forms with said first material (5) a material which is liquid at temperature below said predetermined value, thereby to prevent subsequent restoration of the device to its original condition by reducing the device to a temperature below said predetermined value. The change may be of colour or shape. <IMAGE>

Description

SPECIFICATION Temperature change indicating devices This invention relates to temperature change indicating devices.

More particularly the invention relates to devices for indicating whether a rise in temperature above a predetermined value has taken place.

Such devices find application, for example, in the storage of materials such as drugs, foods and chemicals.

In such an application the indicating device is typically attached to the stored material and is required to indicate whether the temperature of the material has risen above a predetermined storage temperature at any time since the device was attached to the material.

Such devices are suitably constructed so that they undergo an easily detectable change, e.g. a change in visual appearance, when their temperature rises above the predetermined value, and do not regain their original condition when their temperature is subsequently reduced below the predetermined value. One convenient way to achieve this is by utilizing the ability of certain materials to change abruptly from the solid to the liquid state at a predetermined temperature. This property can, for example, be exploited to obtain a change of visual appearance by enclosing a liquid in an elastic container and deforming the container during fabrication i.e. during freezing.

When the liquid melts the container will then adopt the shape of minimum surface area. Alternatively objects may be positioned with a frozen block of material so that the objects change their position when the material melts.

A further possibility is to provide passages or compartments which a material can only enter when the material becomes liquid, e.g. by capillary action.

All these devices however suffer to a greater or lesser extent from the disadvantage that they can be restored to their original visual appearance after operation.

It is thus an object of the present invention to provide a device for indicating a rise in temperature above a predetermined value which avoids this disadvantage.

According to the present invention a device for indicating a rise in temperature above a predetermined value comprises: a quantity of a first material which changes from a solid state to a liquid state and consequently causes a subsequently detectable change in the device when its temperature rises above said predetermined value; and a quantity of a second material which, when said first material becomes liquid, forms with said first material a material which is liquid at temperatures below said predetermined value, thereby to prevent subsequent restoration of the device to its original condition by reducing the device to a said temperature below said predetermined value.

Several devices in accordance with the invention will now be described by way of example with reference to the accompanying drawing in which: Figure 1 is a sectional view of a first device when below its indicating temperature; Figure 2 is a sectional view of the device of Fig. 1 after its temperature has been raised above its indicating temperature; Figure 3 is a sectional view of a second device when below its indicating temperature; and Figure 4 is a sectional view of the device of Fig. 2 after it has been raised above its indicating temperature.

Referring to Fig. 1, the first device to be described comprises a flat sachet of, preferably transparent, plastics material 1 adapted to be attached to a package 3 of a material whose temperature is required to be kept below a predetermined value -T"C during storage of the package.

The sachet 1 contains a first layer of a material 5 which melts at --TOC and second layer of a material 7 which melts at a much higher temperature than -T"C.

If the second package 3 and sachet 1 are raised above -T"C the material 5 melts and dissolves the material 7 to form a liquid having a melting point below -T"C and preferably below the minimum temperature at which it is possible to maintain the enclosure (not shown) in which the package 3 is being stored.

Thus, if at any time during storage the package 3 and its attached sachet 1 have been allowed to rise to a temperature above -T"C, e.g. due to temporary removal from the storage enclosure or temporary failure of the storage enclosure refrigeration system, the contents of the sachet 1 will become and remain a liquid 9, as illustrated in Fig. 2.

With the device of Fig. 1 the liquid condition of the sachet contents if not readily detectable visually, may be detected by touch.

Other devices in accordance with the invention may be arranged to give a more easily detectable change in visual appearance.

Referring now to Fig. 3, one such device comprises a sachet of a thin elastic plastics material 11 containing a rectangular block 13 of material of corresponding properties to the material 5 of Fig. 1, and a rectangular block 13 of material of corresponding properties to the material 5 of Fig. 1, and a rectangular block 15 of material of corresponding properties to the material 7 of Fig. 1.

During fabrication of the sachet the plastics material of the sachet 11 is stretched over the blocks 13 and 15 so that the sachet 11 is of cubical form.

In use of the device, if its temperature rises above -T"C so that the contents of the sa chet 9 becomes a liquid 17, the sachet 11 takes up the shape of minimum possible surface area, i.e. a generally spherical shape, as illustrated in Fig. 4. A rise in temperature of the sachet 11 above -T"C is thus indicated by a change in the visual appearance of the device.

In the devices of Figs. 1 to 4 the two materials 5 and 7 or 13 and 15 are suitably separated by a porous membrane (not shown).

In a modification (not illustrated) of the devices described above by way of example with reference to Figs. 1 to 4 the first and second materials 5 and 7 or 13 and 15 are kept separate by an easily fracturable membrane, or other barrier which therefore, until fractured immediately prior to the device being put into use, prevents the operation of the temperature rise indicating mechanism employed in the device. This avoids the necessity for maintaining the device at a temperature below the predetermined indication value after the device has been fabricated until the device is put into use. This feature forms the subject of a co-pending patent application No.

8427473.

It will be appreciated that other ways of obtaining a temperature rise indication than those described above by way of example may be adopted in a device in accordance with the invention.

For example, an object or particles may be disposed in one or both of the two materials in the sachet during fabrication of the device so as to have a particular position or distribution which is lost when the materials become liquid. Movement of such an object or particles when the materials in the sachet liquify may be arranged to reveal lettering or a symbol, e.g. on the underside of the sachet, previously hidden by the object or particles.

Another possibility is for the contents of the sachet to move to a previously empty compartment of the sachet when it becomes liquid. In further arrangements the two materials in the sachet may be chosen so as together to produce a colour change effect on liquidfying.

It will be appreciated that the choice of materials for a device in accordance with the invention will depend not only on the required indicating mechanism and temperature of indication, but also on the kind of materials whose temperature is to be monitored by the device. For example, for food storage purposes it is necessary that the materials should be non-toxic and not noxious in any way.

Some suitable materials for such an application are an acqueous solution of a salt, such as common salt, calcium chloride or potassium carbonate, and a quantity of the corresponding solid material. Another possibility is carton sugars which melt near -20"C. Other possibilities for the material whose melting point determines the indicating temperature of the device are organic compounds such as:

c2H5-oo C -iCH2)7 COO-C2H5 CgH1g- COO-CH3 HO- ICH2)5- OH Methods of fabrication of a device in accordance with the invention alternative to those described above by way of example will also readily occur to those skilled in the art. For example, an indicating device in accordance with the invention may suitably be fabricated in label-like form by applying the required first and second materials to a carrier using techniques well known in the printing industry. In such devices one or both of the materials may be encapsulated using known micro-encapsulating techniques, i.e. encapsulated as a large number of minute pellets or droplets of material each in a separate enclosure consisting of gelatine or other micro-encapsulant material.

The pellets or droplets are then conveniently applied to a carrier using an ink-jet printing technique.

Such an arrangement finds particular application wherein the first and second materials of the device are kept separate by an easily fracturable membrane until put into use as described above.

Claims (11)

1. A device for indicating a rise in temperature above a predetermined value comprising: a quantity of a first material which changes from a solid state to a liquid state and consequently causes a subsequently detectable change in the device when its temperature rises above said predetermined value; and a quantity of a second material which, when said first material becomes liquid, forms with said first material a material which is liquid at temperatures below said predetermined value, thereby to prevent subsequent restoration of the device to its original condition by reducing the device to a said temperature below said predetermined value.

2. A device according to Claim 1 wherein said change is a change in the visual appearance of the device.

3. A device according to Claim 2 wherein said change is a change in the shape of an enclosure containing said first and second materials.

4. A device according to Claim 2 wherein said change is a colour change.

5. A device according to Claim 2 wherein said change is due to movement of an object or particles contained in one or both of said first and second materials.

6. A device according to Claim 1 or Claim 2 wherein said change comprises a movement of said liquid formed by said first and second materials from one part of an enclosure for said materials to another part.

7. A device according to any one of the preceding claims wherein said first and second materials are contained in separate compartments of an enclosure separated by a porous membrane.

8. A device according to any one of Claims 1 to 6 wherein said first and second materials are contained in separate compartments of an enclosure separated by a barrier adapted to be fractured when the device is brought into use.

9. A device according to any one of the preceding claims wherein said first and second materials are in the form of a large number of pellets or droplets encapsulated by a microencapsualtion process.

10. A method of fabricating a device according to Claim 9 wherein said pellets or droplets are applied to a carrier using an ink-jet printing technique.

11. A device for indicating a rise in temperature above a predetermined value substantially as hereinbefore described with reference to Figs. 1 and 2 or Figs. 3 and 4.