GB1604649A - Temperature indicator device for providing visual indication as to whether a predetermined temperature has been exceeded for a predetermined time - Google Patents

Description

(54) TEMPERATURE INDICATOR DEVICE FOR PROVIDING VISUAL INDICATION AS TO WHETHER A PREDETERMINED TEMPERATURE HAS BEEN EXCEEDED FOR A PREDETERMINED TIME (71) I, GEORGE ALFRED KIRK HAM, a British subject of 12, Dorincourt, Talbot Road, Oxton, Birkenhead, Merseyside, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to temperature indicators of the type adapted to give a visible indication as to whether an article, such as an item of frozen food, has been warmed to a temperature exceeding a predetermined value for a predetermined time.

Frozen foods are subject to deterioration if defrosted and refrozen. This can happen before purchase by the customer so that the customer may buy frozen food which has been defrosted and subsequently refrozen while under the impression that the food is still in its initially frozen condition.

Many devices have been proposed previously for indicating when an article has become defrosted, even though it has been subsequently refrozen, but such devices have been bulky, complicated and relatively expensive to produce.

It is an object of the present invention to provide such a device which avoids the problems of the known devices and is essentially simple and cheap to manufacture.

In accordance with the present invention there is provided a device for providing a visual indication as to whether a predetermined temperature has been exceeded for a predetermined time comprising indicator and initiator substances which are disposed separately within an at least partially transparent or translucent container and which are such that, when combined, result in a colour change in the indicator substance visible externally of the container, said substances being located along a path capable of carrying liquid, said path being constituted by an absorbent material member which is arranged to absorb free liquid, moisture or water vapour from the ambient surroundings outside said container and conduct the resulting liquid along said path so as to take up initiator substance and carry same to the indicator substance to thereby bring about said externally visible colour change.

In one embodiment of such an indicator device the absorbent material member extends out of the container, the part of said absorbent material member within the container carrying the indicator and the initiator substances at longitudinally spaced locations thereon.

Preferably the absorbent material member is in the form of a fiat strip, the container being formed by opposed layers of liquid proof tape of waterproof paper from which an exposed "tail" of the absorbent strip extends.

Alternatively however, the absorbent material member may carry the indicator and initiator substances on two opposite surfaces respectively, whereby, when the absorbent material member comes into contact with a liquid, the indicator and initiator substance are brought together by the passage of liquid along the liquid path formed by the thickness of the absorbent material, and thereby to effect the externally visible colour change.

In this latter case, it is preferred if the surface of the absorbent material carrying the indicator is permanently covered by a transparent or translucent waterproof layer whereby liquid or moisture thereof is prevented from entering the material via that surface.

In order to enable the time to be preset between liquid being absorbed outside the enclosed space and a colour change occuring, the absorbency properties of the absorbent material member can be predetermined, either over its whole volume or in one or more particular regions.

In another embodiment of the invention the initiator is, or is in the proximity of, a deliquescent substance which, above a predetermined temperature and in sufficiently moist conditions absorbs vapour and liquifies to provide a carrier whereby the initiator is carried along the liquid path to the indicator for the purpose of bringing about said colour change.

Normally, the device in accordance with this latter embodiment, would be used on a frozen food item exposed to external atmospheric conditions or exposed to the atmospheric conditions within the packaging of the food item whereby the vapour absorbed by the deliquescent material above said predetermined temperature is water vapour.

Below such predetermined temperature, which corresponds to the eutectic point of a eutectic mixture of ice and the deliquescent material chosen a dynamic equilibrium is reached such that no net quantity of vapour is absorbed and no change occurs. However, above said predetermined temperature, water vapour is absorbed which eventually dissolves the deliquescent substance to provide the liquid carrier which is itself absorbed by the absorbent material to bring together the initiator and indicator substances.

The deliquescent substance is conveniently in the form of solid crystals mixed with the initiator material but it is equally possible for the deliquescent substance itself to form either the initiator or the indicator.

The initiator, indicator, deliquescent substance and absorbent material may all be housed in said container which has openings for the passage of liquid or vapour into its interior, the openings being positioned so that such liquid or vapour must contact the deliquescent material upon entering the container, preferably before contacting the absorbent material. Advantageously, apart from said openings, the container is otherwise air tight and made from a non-absorbent, at least partially transparent material. In order to prevent premature activation of the device, the openings might be covered before fitting of the device in its operative position on the food or other item to be monitored or the whole device kept in a dry, waterproof container.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic sectional side view of one embodiment of a device in accordance with the invention, taken on the line I-I of Fig. 2; Fig. 2 is a top plan view of the device of Fig. 1; Fig. 3 is a diagrammatic sectional side view of a second embodiment of a device in accordance with the invention; Fig. 4 is a top plan view of a third embodiment in accordance with this invention; and Fig. 5 is a sectional side view of the embodiment of Fig. 4; Fig. 6 is a diagrammatic sectional view of a fourth embodiment of a device in accordance with the present invention; Fig. 7 is a diagrammatic sectional side view of a fifth embodiment of an indicator device in accordance with the present invention; Fig. 8 is a plan view from below of the device of Fig. 7; Fig. 9 is a diagrammatic sectional side view of a sixth embodiment of an indicator device in accordance with the invention fitted with a packing strip; and Fig. 10 is a diagrammatic sectional side view of part of seventh embodiment of an indicator device in accordance with the invention.

The device illustrated in Figs. 1 and 2 includes a strip of absorbent material 10, e.g.

absorbent paper, which can be considered to comprise a plurality of distinct zones, A, B, C and D. Zone A is plain with no special additives and normally would be white or substantially colourless. Zone B is also basically colourless or white but contains a colourless or white substance (Y) within the absorbent material whose nature is described further below. The latter substance (Y) is referred to as tbe "initiator". Zone C is again plain and forms a gap which separates Zone B from Zone D. Zone D is referred to as the "indicator" zone and contains a substance (X) within the absorbent material together with an indicator dye which has a special readily identifiable colour, e.g. bright blue, under normal atmospheric conditions.

Zone D, zone C zone B and part of zone A are isolated from the atmosphere by two strips of transparent waterproof adhesive tape 12, 14 which are placed sticky-side to stickyside so as to form an air-tight pocket or vessel for these zones. It will be noted that the free end portion (right hand portion in Fig. 1) of zone A projects from the latter vessel. This may be as shown in Fig. 1 in which only one side is exposed. Alternatively, the top layer 12 of tape can terminate substantially at the same region as the bottom layer 14 so that the zone A part of the absorbent strip has a wholly exposed 'tail".

The part of zone A between the junction with zone B and the start of the exposed portion forms a sub-zone E referred to as the "throat".

The basic operation of the device described above is as follows. In the normal state, the absorbent paper is dry and the device is in an equilibrium condition with the indicator exhibiting its initial colour (blue). Should, however, the exposed part of zone A of the absorbent material come into contact with a liquid (e.g. water), the water passes along the material of zone A and into the throat zone E. Eventually this water reaches zone B where it picks up some of the colourless substance (Y) and carries this along in solution through gap C and into zone D where it comes into contact with the indicator dye and the substance X.Substances X and Y are chosen such that on mixing in solution a chemical reaction occurs whereby the substance Y neutralises the substance X and, thereafter, an excess of substance Y causes the indicator dye to undergo a significant colour change, e.g. from blue to red.

The chemical reaction can be considered to be: X+Y4A+B When substance Y is carried into the blue zone, X is removed by the reaction.

Now X and Y are chosen such that in the presence of an excess of substance X, the indicator is blue but in the presence of an excess of Y, the indicator changes to red. Therefore, as Y is carried into the blue zone by the water, X is gradually removed and the indicator changes from blue to red.

The net visual effect is that, when water passes into the waterproof vessel formed by the tapes 12, 14, the blue zone is caused to turn red. This can only occur, therefore, when free liquid water is present in contact with the exposed "tail" of zone A. When the water is frozen, no absorbtion by zone A can take place and so no colour change can occur.

In one embodiment for example, X might be an alkali such as sodium carbonate and Y might be an acid such as citric acid, the indicator being litmus.

The requirement for the presence of substance X is to stabilise the indicator to prevent a spurious colour change occuring due to special environmental conditions. The substance X could therefore be omitted in some cases although its presence is preferable.

Clearly any suitable substance Y and indicator dye can be used and the invention is not limited to the use of any particular substances in this respect.

An important aspect of the device is that the time lag between the exposed part of zone A coming into contact with water and the water carrying substance Y reaching the indicator dye can be accurately controlled in a variety of ways. For example, this time lag can be controlled by the length and/or thickness of the throat zone E. The properties of the absorbent material in the region of the throat zone E can be chosen to have absorbent properties such that water travels along it as quickly or as slowly as required.

The vessel can be formed in ways other than that described above. Fig. 3 illustrates a device in which one side of the vessel is formed by packaging material 16, the other side being formed by a strip of adhesive tape 18 so as to leave an exposed tail portion 20 of zone A.

In other embodiments, the lower tape portion 18 of the Fig. 3 embodiment could be sprayed on to form a coverging layer for zones B, C, D and part of zone A.

In actual use of the device, it is fitted to a food product so that at least the indicator portion is visible externally of the packaged item. The food item is then frozen. As long as it remains frozen no change in the indicator occurs. Should, however, the food item be defrosted for more than the lag period determined above, the water which inevitably forms on a defrosted object is absorbed by the exposed portion of zone A and the colour change occurs giving a visible and irreversible indication that defrosting has taken place.

As is illustrated in Figs. 4 and 5 another embodiment is feasible which is in the form of a tab, for example of circular form. The absorbent material can comprise a disc having waterproof discs of smaller diameter on either side to form the "vessel", the disc containing the substance Y (substance X if required) and the dye. In Figs. 4 and 5 similar parts are given the same reference numerals and letters as those used in Figs. 1, 2 or 3.

As mentioned above, the device can be part of the packaging itself or may be included within or on the packaging as a separate item.

The device of Fig. 6 comprises a piece 58 of absorbent material, e.g. a strip or disc of absorbent paper, having applied to one surface thereof a layer or coating 60 of "initiator" substance Y. The other surface of the absorbent member 58 has applied to it a layer or coating 62 of "indicator" material preferably together with stabilising substance X. The layers 60 and 62 can be applied to the absorbent material 58 by any suitable method, e.g. by painting or spraying.

The member 58 is enclosed within a waterproof vessel or enclosed space 63 formed by opposed strips 64 and 66 of waterproof tape adhered together at their edges. A further strip 68 of absorbent material extends out of the latter enclosed space 63 to form a free "tail" portion 68a, the portion 68b of this strip within the enclosed space 63 being arranged to contact the absorbent material 58 via the coating or layer 60 containing the initiator Y.

Thus, in use, when free water or moisture comes into contact with the tail portion 68a outside the enclosed space 63, this is absorbed by the tail portion and carried into the enclosed space 63 where it can then travel through the absorbent member 58, having picked up some of the initiator substance Y from layer 60, to eventually reach the indicator and provide the necessary conditions for the colour change to occur.

The latter embodiment has the advantage that it is easier to provide a controlled time delay between the water or moisture being initially picked up by the tail portion 68a and the initiator Y reaching the indicator to effect a colour change. As descrbed with reference to the embodiments illustrated in Figs. 1 to 5, this time delay can be determined by control of the properties of the materials carrying the water through the device, e.g. the rate of absorbence of liquid through a portion N of the strip 68 within the enclosed space 63.

The overall shape of the device of Fig. 6 in plan can be chosen as required, e.g. the parts 64, 66 defining the enclosed space 63 can be circular, rectangular or any other shape desired. The tail portion extending from the enclosed space 63 can then be elongate.

Alternatively, the tail portion 68a could be annular and extend around part of, the whole of, or substantially the whole of, the vessel defining the enclosed space 63 as in the embodiment of Figs. 4 and 5.

In order for the aforegoing devices to work satisfactorily, it is necessary for free aqueous liquid to be present for the two substances to be brought together. With many frozen food items, such as frozen poultry for example, thawing is almost inevitably accompanied by the formation of adequate water or moisture to activate the aforegoing devices.

However, the defrosting of many other frozen food items is not accompanied by the formation of significant or adequate quantities of free aqueous liquid although an increase in the amount of water vapour in the surrounding atmosphere would normally occur.

The devices illustrated in Figs. 7 to 10 get around this problem, however, by the use of a deliquescent substance which can absorb the relatively small amounts of water vapour available from relatively dry frozen food articles or from the surrounding environment. Thus when the temperature rises beyond a predetermined limit this absorbed water vapour dissolves the deliquescent substance and the resulting liquid forms the carrier for the initiator.

In more detail, the device of Figs. 7 and 8 comprises an absorbent member 110 and a quantity of crystals 112 of a mixture of a deliquescent material, such as ammonium nitrate, and initiator material such as citric acid, the crystals 112 lying in contact with one surface 114 of the absorbent member 110. The absorbent member 110 and crystals 112 are disposed within a container 116 which, in this example, is constructed from a pair of rectangular strips 118, 120 of nonabsorbent, water-proof, material, such as transparent or translucent plastics sheet, which are welded together around all four sides in an air-tight manner, as indicated at 122.

In order to provide a path for the entry of water vapour into the interior of the container 116, the strip 120 contains a plurality of holes 124 disposed such that water vapour entering these holes passes through the crystals 112.

The absorbent member 110 can be, for example litmus paper (blue litmus when the initiator is citric acid) or absorbent material having a litmus coating which serves as the "indicator". Preferably, the aforementioned stabilising substance X, such as sodium carbonate, is also carried by the absorbent member to prevent the indicator changing colour spuriously, due for example to abnormally acidic atmospheric conditions.

In use, for a given deliquescent substance forming part of the crystals 112, there is a temperature below which no net quantity of water vapour is absorbed from the surrounding atmosphere, this temperature corresponding to the eutectic point of a mixture of the substance in question with ice. In the case of ammonium nitrate this temperature is approximately - 160C. Thus, as long as the temperature is maintained below - 16 C, for example at a conventional domestic freezer temperature of approximately --220C, no net quantity of water vapour is absorbed by the deliquescent substance and no change occurs.

However, if the temperature is permitted to rise above - 160C, an amount of water vapour which increases as the temperature rises above - 160C, is absorbed by the deliquescent substance. Eventually, the latter substance dissolves in the water vapour it has absorbed to form a solution which also contains dissolved "initiator", that is in this instance dissolved citric acid. The liquid so formed is then itself absorbed by the absorbent material 110 so as to bring the citric acid "initator" into contact with the litmus "indicator" to thereby bring about a colour change in the litmus from blue to red which is externally visible through the transparent or translucent wall 118 of the container 116.

The aforegoing device does not therefore require free water to be present externally for a temperature change to be detected and indicated although, of course, if free water is present the device will react all the more quickly.

One possible problem with the aforegoing device which can occur where the crystals are in contact with a large area of the absorbent material is that spurious liquidification of individual crystals of the deliquescent substance, even at low temperatures in the region of - 160C, can lead to the absorbent material adopting a spotted appearance which can be misleading. In order to avoid this possibility, in the embodiment of Fig. 9 (wherein identical parts have been given the same reference numerals) there are two strips of absorbent material 130, 132 which are separated over the majority of their side surfaces by a strip 134 of non-transparent, non-absorbent material. There is contact between the two strips of absorbent material 130, 132 only at edge regions 136, 138.Thus, it does not matter if the lower absorbent material strip 132 becomes spotted since it cannot be seen through the strip 134. When, however, the deliquescent substance goes into solution and there is a relatively large amount of liquid present, this is transferred to the upper absorbent material strip 130 via the edge contact regions 136, 138 to effect the principal, externally visible colour change.

An alternative way of preventing spurious "spotting" of the absorbent material is to make the latter material somewhat thicker than in Figs. 7 to 9. This possibility is illustrated in Fig. 10 wherein a strip 140 of absorbent material has a thickness X, most of which is composed of plain absorbent material not carrying within it any indicator material, the latter material being associated only with an upper region Y. A simple way of forming such a strip is to apply a layer 142 of litmus to a thicker layer 144 of plain absorbent material. Liquification of individual crystals then provides insufficient liquid to pass through the thickness X-Y to reach the litmus layer.

The aforegoing devices are all temperature dependent in that they react to the environmental temperature being raised above the eutectic point, in this case above -16"C.

Obviously, the reaction time of the devices is not instantaneous and raising the temperature above - 160C for a short period, e.g. a few minutes, followed by immediate re-cooling will not be indicated. However, the device does exhibit a memory type characteristic in that any water absorbed during such short periods is retained and cummulatively stored so that on next warming above - 160C the device reacts more quickly than before by virtue of the stored liquid content.

In all such devices, it is of course necessary to keep water vapour away from the deliquescent substance at normal room tem peratures and before the device is applied to the food item with which it is to be associated. This can conveniently be achieved as indicated in Figs. 7 and 9 by a protective layer 146 which normally blocks the holes 24 but which can be peeled off, or otherwise removed, immediately prior to application to the food item or by storing the device immediately after manufacture in a dry, waterproof container.

The aforegoing devices can be manufactured in a wide variety of ways. However, one convenient way is to form the lower strips 120 of non-absorbent material as depressions in a continuous strip of such material, the crystals and absorbent material being placed into such depressions prior to application of the upper non-absorbent strip 118. The joints 122 can then be formed by heat sealing and the individual devices separated.

WHAT I CLAIM IS: 1. A device for providing a visual indication as to whether a predetermined temperature has been exceeded for a predetermined time comprising indicator and initiator substances which are disposed separately within an at least partially transparent or translucent container and which are such that, when combined, result in a colour change in the indicator substance visible externally of the container, said substances being located along a path capable of carrying liquid, said path being constituted by an absorbent material member which is arranged to absorb free liquid, moisture or water vapour from the ambient surroundings outside said container and conduct the resulting liquid along said path so as to take up initiator substance and carry same to the indicator substance to thereby bring about said externally visible colour change.

2. A device as claimed in claim 1 in which part of the wall of the container is formed by a portion of the packaging material of an article whose temperature is to be monitored, which portion is at least partially transparent or translucent.

3. A device as claimed in claim 1 or 2, in which said absorbent material member extends out of the container, the part of said absorbent material member within the container carrying the indicator and initiator substances at longitudinally spaced locations on the absorbent material.

4. A device as claimed in claim 3, in which the absorbent material member is in the form of a flat strip, the vessel being formed by opposed layers of liquid proof tape or waterproof paper from which an exposed tail of the absorbent strip extends.

5. A device as claimed in claim 4, in which the indicator is carried by the absorbent strip in a first region thereof at the base of the vessel and the initiator is carried by said absorbent strip in a second region thereof adjacent a throat of the vessel.

6. A device as claimed in claim 3, in which the vessel is in the form of opposed layers of liquid proof tape or waterproof paper, between which there is disposed the absorbent material member, a substantial portion of the edges of the absorbent material member being exposed to the outside of the vessel.

7. A device as claimed in claim 6, in which the indicator is carried by the absorbent

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (1)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   in the embodiment of Fig. 9 (wherein identical parts have been given the same reference numerals) there are two strips of absorbent material 130, 132 which are separated over the majority of their side surfaces by a strip 134 of non-transparent, non-absorbent material. There is contact between the two strips of absorbent material 130, 132 only at edge regions 136, 138. Thus, it does not matter if the lower absorbent material strip 132 becomes spotted since it cannot be seen through the strip 134. When, however, the deliquescent substance goes into solution and there is a relatively large amount of liquid present, this is transferred to the upper absorbent material strip 130 via the edge contact regions 136, 138 to effect the principal, externally visible colour change.

   An alternative way of preventing spurious "spotting" of the absorbent material is to make the latter material somewhat thicker than in Figs. 7 to 9. This possibility is illustrated in Fig. 10 wherein a strip 140 of absorbent material has a thickness X, most of which is composed of plain absorbent material not carrying within it any indicator material, the latter material being associated only with an upper region Y. A simple way of forming such a strip is to apply a layer 142 of litmus to a thicker layer 144 of plain absorbent material. Liquification of individual crystals then provides insufficient liquid to pass through the thickness X-Y to reach the litmus layer.

   The aforegoing devices are all temperature dependent in that they react to the environmental temperature being raised above the eutectic point, in this case above -16"C.

   Obviously, the reaction time of the devices is not instantaneous and raising the temperature above - 160C for a short period, e.g. a few minutes, followed by immediate re-cooling will not be indicated. However, the device does exhibit a memory type characteristic in that any water absorbed during such short periods is retained and cummulatively stored so that on next warming above - 160C the device reacts more quickly than before by virtue of the stored liquid content.

   In all such devices, it is of course necessary to keep water vapour away from the deliquescent substance at normal room tem peratures and before the device is applied to the food item with which it is to be associated. This can conveniently be achieved as indicated in Figs. 7 and 9 by a protective layer 146 which normally blocks the holes 24 but which can be peeled off, or otherwise removed, immediately prior to application to the food item or by storing the device immediately after manufacture in a dry, waterproof container.

   The aforegoing devices can be manufactured in a wide variety of ways. However, one convenient way is to form the lower strips 120 of non-absorbent material as depressions in a continuous strip of such material, the crystals and absorbent material being placed into such depressions prior to application of the upper non-absorbent strip 118. The joints 122 can then be formed by heat sealing and the individual devices separated.

   WHAT I CLAIM IS:

   1. A device for providing a visual indication as to whether a predetermined temperature has been exceeded for a predetermined time comprising indicator and initiator substances which are disposed separately within an at least partially transparent or translucent container and which are such that, when combined, result in a colour change in the indicator substance visible externally of the container, said substances being located along a path capable of carrying liquid, said path being constituted by an absorbent material member which is arranged to absorb free liquid, moisture or water vapour from the ambient surroundings outside said container and conduct the resulting liquid along said path so as to take up initiator substance and carry same to the indicator substance to thereby bring about said externally visible colour change.

   2. A device as claimed in claim 1 in which part of the wall of the container is formed by a portion of the packaging material of an article whose temperature is to be monitored, which portion is at least partially transparent or translucent.

   3. A device as claimed in claim 1 or 2, in which said absorbent material member extends out of the container, the part of said absorbent material member within the container carrying the indicator and initiator substances at longitudinally spaced locations on the absorbent material.

   4. A device as claimed in claim 3, in which the absorbent material member is in the form of a flat strip, the vessel being formed by opposed layers of liquid proof tape or waterproof paper from which an exposed tail of the absorbent strip extends.

   5. A device as claimed in claim 4, in which the indicator is carried by the absorbent strip in a first region thereof at the base of the vessel and the initiator is carried by said absorbent strip in a second region thereof adjacent a throat of the vessel.

   6. A device as claimed in claim 3, in which the vessel is in the form of opposed layers of liquid proof tape or waterproof paper, between which there is disposed the absorbent material member, a substantial portion of the edges of the absorbent material member being exposed to the outside of the vessel.

   7. A device as claimed in claim 6, in which the indicator is carried by the absorbent

   strip in a first region thereof disposed centrally in the vessel and the initiator is carried by said absorbent strip in a second region thereof disposed adjacent a periphery of the vessel.

   8. A device as claimed in claim 1 or 2, in which the indicator is disposed in a first region of the absorbent material member adjacent one surface thereof and the initiator is disposed in a second region adjacent another surface of the absorbent material member.

   9. A device as claimed in claim 8, in which the surface of the absorbent materal member adjacent the first region is disposed against a transparent or translucent waterproof layer whereby liquid or moisture thereof is prevented from entering the material via that surface.

   10. A device as claimed in claim 5 or any of claims 7 to 9, in which the first and second regions of the absorbent material member are separated by a region thereof which is devoid of indicator and initiator.

   11. A device as claimed in any of claims 3 to 10, in which either the initator or the indicator or both are sprayed or painted or applied by other means onto respective surfaces of the absorbent material member.

   13. A device as claimed in any preceding claim in which there is disposed in said path a region of predetermined absorbency whereby there is provided a controlled time delay between the formation of liquid or moisture and its indication on the temperature indicator.

   14. A device as claimed in claim 1 or 2, in which the initiator is, or is in the proximity of, a deliquescent substance which, above a predetermined temperature and in sufficiently moist conditions, absorbs vapour and liquifies to provide a carrier, whereby the initiator is carried along said path to the indicator for the purpose of bringing about said colour change.

   15. A device as claimed in claim 14, in which the initiator, indicator, deliquescent substance and absorbent material are all disposed in the vessel, which vessel has openings for the passage of liquid, moisture or vapour into its interior, the openings being positioned so that such liquid, moisture or vapour must contact the deliquescent substance upon entering the container before contacting the absorbent material.

   16. A device as claimed in claim 15, in which the deliquescent substance is contiguous with only a first surface of the absorbent material member, and in which the indicator is sprayed or painted or applied by other means onto only a second surface of the absorbent material member, which surface is separated from the first surface by the thickness of the absorbent material member, whereby spurious discolouring of the absorbent material member in the region of the indicator is avoided.

   17. A device as claimed in claim 15, in which the absorbent material member carrying the indicator is separated from the deliquescent substance by a further absorbent material member and by a piece of nonabsorbent material disposed between said first mentioned absorbent material member and said further absorbent material member, said path comprising at least a part of said absorbent material and a region of contact between the latter and the piece of absorbent material, whereby spurious discolouring of the absorbent material in the region of the indicator is avoided.

   18. A device as claimed in any preceding claim in which the indicator is provided with a stabilising substance, whereby spurous discolouring of the absorbent material in the region of the indicator, because of special environmental conditions, is avoided.

   19. A device as claimed in any preceding claim in which the indicator substance is blue litmus and the initiator substance is citric acid.

   20. A device as claimed in claims 18 and 19, in which the stabilising substance is sodium carbonate.

   21. A device as claimed in claim 14, or as claimed in any of claims 15 to 20 when appendant to claim 14, in which the deliquescent substance is ammonium nitrate.

   22. A device substantially as hereinbefore described with reference to and as illustrated in Figs. 1 to 3 of the drawings.

   23. A device substantially as hereinbefore described with reference to and as illustrated in Figs. 4 and 5 of the accompanying draw ings.

   24. A device substantially as hereinbefore described with reference to and as illustrated in Figs. 7 and 8 of the accompanying drawings.

   25. A device substantially as hereinbefore described with reference to and as illustrated in Figs. 8 and 9 of the accompanying drawings.

   26. A device substantially as hereinbefore described with reference to and as illustrated in Figs. 8 and 10 of the accompanying drawings.