GB2218242A

GB2218242A - Liquid sensor

Info

Publication number: GB2218242A
Application number: GB8908660A
Authority: GB
Inventors: Arthur Leonard Lewis
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-04-19
Filing date: 1989-04-17
Publication date: 1989-11-08
Also published as: GB8901111D0; GB8809195D0; GB8908660D0

Abstract

The liquid sensor 10 comprises upper and lower sheet electrodes 12, 14 attached to upper and lower sheets 16, 18 of insulating material. The insulating sheets are attached together to insulate the sheet electrodes. The upper electrode and the insulating sheets have perforations which are aligned to allow electrically conducting fluid to form an electrical connection between the electrodes, through the perforations. The insulating sheets are attached only along attachment lines 20 around the periphery of the perforations. <IMAGE>

Description

Liquid Sensor The present invention relates to liquid sensors and in particular, but not exclusively to sensors for detecting water.

Many circumstances arise in which it is desirable to sense the presence of liquids, in order to detect leaks, pipe bursts or the like. Sensors have therefore been proposed which respond to the presence of liquid to produce an electrical signal which causes an alarm to be triggered.

One proposal provides a flexible blanket formed by a sandwich structure of two sheet electrodes and an intermediate layer of absorbent material. The upper electrode is perforated to allow liquid to be absorbed by the middle layer and thus to alter the resistance or capacitance between the electrodes, and allow for electrical detection of the liquid.

It has been found that, once the proposed device has absorbed liquid and triggered an alarm, the sensor must be out of action for a considerable period of time in order to allow the middle absorbent layer to dry out. Drying is hindered because liquid can only leave the absorbent layer through the relatively small surface area of the perforations in the upper electrode.

It is an object of the present invention to obviate or mitigate this or other disadvantages of the prior art.

According to the invention there is provided a liquid sensor comprising first and second electrodes attached respectively to first and second layers of insulating material, the insulating layers being attached together to form a unitary structure in which the electrodes are electrically insulated one from the other, the insulating layers being attached together at at least one attachment position, there being at least one other position in which the insulating layers are not attached.

Preferably the first electrode has a first edge beyond which the second electrode projects and is exposed. The first electrode and the insulating layers each having at least one gap formed therein, the gaps being aligned electrically conducting fluid to form an electrical connection from the first electrode through the gaps to the second electrode.

The first electrode is preferably formed by a sheet of electrically conducting material, the or each gap being formed between adjacent regions of the sheet.

At least one of the insulating layers is preferably formed by a sheet of electrically insulating material, the or each gap being formed between adjacent regions of the sheet.

Preferably at least one of the gaps is a perforation wholly bounded by the electrode or insulating layer in which the perforation is formed.

The first electrode preferably has a second plurality of gaps formed therein in addition to the gap or gaps as aforesaid, the second gaps allowing the sensor to be penetrated by a fixing nail located in one of the second gaps, without an electrical connection being established between the electrodes.

The second electrode may have gaps formed therein and aligned with the gaps in the insulating layers and the first electrode, the gaps in the second electrode being closed by a material through which liquid in the gaps is visible.

Preferably the closing material is a sheet which extends across substantially the whole area of the second electrode and the gaps formed therein.

Preferably the insulating layers are attached along one or more attachment lines which run along the edge or edges of the electrodes, whereby liquid is prevented from passing inbetween the sheets.

The second electrode is preferably a sheet of electrically conducting material.

At least one of the insulating layers or the first electrode is preferably formed by at least one elongate strip of material laid side by side in the sensor to form gaps between adjacent strips.

The sensor may further comprise a pad laid against the first electrode and of a material which allows liquid to penetrate the pad, the first electrode and the insulating layers being sufficiently thin to allow an electrically conducted path to be formed between the electrodes through liquid within the pad.

The pad is preferably lightly bonded to the first sheet electrode to allow the pad to be removed and replaced.

Preferably the first electrode is formed from a metal foil. Preferably the insulating layers are formed from sheets of plastics material. Preferably the sheets are attached by welding. Preferably the insulating layers and the electrodes are all flexible.

Embodiments of sensors according to the present invention, and various possible modifications and alterations will now be described in more detail, by way of example only and with reference to the accompanying drawings in which: Fig. 1 is a plan view of the sensor, partially cut away; Fig. 2 is a schematic section along the line II-II in Fig. 1; and Fig. 3 is a schematic plan view of an alternative embodiment.

The drawings are schematic and not to any particular scale. In particular, the vertical dimension shown in Fig. 2 are highly exaggerated for clarity.

Fig. 1 shows a liquid sensor 10 comprising first (upper) and second (lower) electrodes 1 2 14 attached respectively to first (upper) and second (lower) layers 16,18 of insulating material. The insulating sheets 16,18 are attached together to form a unitary structure in which the electrodes 12,14 are electrically insulated one from the other. The sheets 16,18 are attached at attachment positions 20, separated by regions 22 in which the sheets 16,18 are not attached.

The first electrode has gaps 24 formed therein, and the sheets 16,18 have gaps 26 aligned with the gaps 24 electrically conducting fluid at the edges of the sensor may form an electrical connection from the first electrode 12 through the gaps 24,26 to the second electrode 14, as will be described.

In more detail, the sensor 10 comprises a first, upper electrode sheet 12 formed from metal foil and perforated by circular perforations located at points on a square grid. The centre of each perforation 24 is separated from the centre of its nearest neighbour by about 7 centimetres, and each perforation has a radius of about 2.5 centimetres.

The metal foil electrode 12 is glued over substantially the whole of its area to the upper insulating sheet 16 by a non-deliquescent glue. The insulating sheet 16 is also perforated by circular perforations 26 arranged on the same grid as the perforations 24 and aligned to be concentric with the perforations 24.

The lower insulating sheet 18 is identical to the sheet 16 and the perforations 26 in both are aligned to be concentric.

The insulating sheets 16,18 are made of an electrically insulating, weldable material such as polyvinylchloride or another plastics material. The sheets 16,18 are welded together around the entire periphery 20 of each perforation 26. These welds 20 provide seals which prevent liquid passing in between the sheets 16,18, from the perforations 26.

The lower electrode 14 is of metallic foil and is attached to the face of the sheet 18 remote from the sheet 16. In the embodiment shown, the sheet 14 has perforations 28 concentric with the perforations 24,26.

These perforations 28 are sealed by a lowermost, waterproof sheet 30 of transparent and electrically insulating material such as polyvinylchloride.

Non-deliquescent adhesive is used to attach the sheet 30 to the sheet electrode 14.

A pad 32 of absorbent material is placed against the upper surface of the sheet electrode 12. The pad 32 may be a slab of uniform material or may comprise an upper layer of highly absorbent material such as paper and a lower layer such as a sponge or sponge-like structure. The pad 32 is lightly attached to the sheet electrode 12 by spots of non-delequescent glue 34. The pad may be impregnated with salts or other materials which may dissolve to render a fluid electrically conductive. Similarly, the pad 32 may be impregnated by a material which causes staining when dissolved by the fluid to be detected. This material may be potassium permanganate.

The sensor 10 may be installed and used in the following manner. The sensor is first cut to size for the task. This is done by two cutting operations using scissors, for instance. In the first operation, the upper sheet electrode 12 and the upper insulating sheet 16 are cut together, with the lower blade of the scissors passing between the insulating sheets 16 in regions 22 between the welds 20 in which sheets 16,18 are not attached. A second cutting operation then cuts the lower insulating sheet 18, the lower sheet electrode 14 and the sheet 30, preferably along a line staggered from the first cut, to form a stepped edge as shown at the left of Fig. 2. The double cutting operation and the stagger minimise the risk of burrs of metal foil coming into contact across the cut edge to short the electrodes 12,14 together.

Once the sensor has been cut to shape, a similarly shaped pad 32 is attached by glueing to the upper insulating sheet 16. Alternatively, the sensor may be supplied with the pad 32 already attached, the pad being cut to size in the first cutting operation.

Finally, electrical connections (not shown) are made from the sheet electrodes 12,14 to suitable circuitry whose function will be clear from the following.

During normal use, in the absence of liquid, the insulating sheets 16,18 insulate the electrode 12 from the electrode 14 so that no current can pass between them. This high resistance state can be detected by circuitry to which the electrodes 12,14 are connected.

If liquid appears, for instance by virtue of a pipe burst or other leak, this will initially be absorbed by the upper layer of the pad 32. A small amount of liquid, such as drips of condensation, will be absorbed by this upper layer and then eventually evaporate without having triggered an alarm.

In the event of a leak, the upper layer will eventually become saturated and liquid will begin to pass into the sponge layer of the pad 32. The liquid will dissolve salts and staining materials present.

If the structure of the sensor is made sufficiently thin, the liquid in the sponge layer of the pad 32 can make contact with the lower sheet electrode 14 through the perforations 26 and thereby provide an electrical connection between the two sheet electrodes 12,14. Liquid may leave the pad 32 to fill up the perforations. This drop in electrical resistance is detected by associated circuitry which triggers an alarm.

Liquid which passes into the perforations cannot enter the space between the insulating sheets 16,18, because of the welds 20. The transparent sheet 30 and the perforations 28 in the lower sheet electrode 14 and the staining agents in the pad 32 allow the location of the leak to be determined by eye, from beneath the sensor. The sheet 30 also prevents liquid passing through the sensor, to provide initial protection to items beneath.

The whole pad 32 or the dampened part of it can be removed from the sensor after use and the sensor can then be wiped clean and dry, before a fresh pad 32 or part is installed. The sensor can therefore be brought back into operation very quickly after an alarm has been triggered.

A simpler version of the sensor 10 is also envisaged. This omits the pad 32 and the sheet 30 and also omits the perforations in the lower sheet electrode 14 which therefore forms a liquid-proof barrier layer. In this arrangement, the liquid to be detected enters the perforations and fills them until a connection is made between the two sheet electrodes 12,14. This requires only a small amount of liquid, by virtue of the thinness of the structure. Again, liquid cannot enter the space between the insulating sheets 16,18, because of the welds 28. This embodiment can be wiped dry for re-use.

Fig. 3 shows a small sensor constructed in an alternative manner. Numerals corresponding to those used in Figs. 1 and 2 are suffixed A in Fig. 3. The sensor 10A consists of an upper electrode 12A formed from sheet metal foil, a lower electrode 14A, also formed from sheet metal foil, and two intervening insulating layers only one of which, 16A, is visible in Fig. 3.

Elongate gaps 24A are formed between adjacent regions of the upper electrode 12A. Aligned with these are elongate gaps 26A formed in the insulating layers 16A. The gaps 24A, 26A are aligned so that the lower electrode 14A is exposed.

It can be seen at the right hand side of Fig. 3 that adjacent strips of the electrode 12A may be connected at the edge of the sensor 10A.

Alternatively, as shown at the left of Fig. 3, adjacent regions may be left unconnected. If the right hand edge of the sensor 10A was at the broken line 42, the adjacent portions would be left unconnected at both ends. It would then be necessary to provide separate electrical connections to each portion of the electrode 12A. It is envisaged that in this arrangement, the sensor 10A may be formed by laying strips of insulating material on a sheet 14A, to form the lower insulating layers. The upper insulating layer 16A is then made in strips on top of the lower layer, and finally, strips of electrode material are laid on the insulating layers to form the upper electrode layer 12A. Long, relatively narrow sensors can be manufactured in this way, for use around or underneath pipes, for instance.

The sensor could be constructed from layers each formed by a single strip. In this case, the sensor would be bounded at the broken lines 44 (Fig. 3). Electrical connection between the electrodes would occur only at the edges of the strips. The edge of the lower electrode could project beyond the edge of the upper electrode, and be exposed.

It may be of assistance in attaching the sensor 10A in its working position if additional perforations 40 are provided along the regions of the upper layer 12A. The perforations 40 allow nail or screw to be used to secure the sensor 10A in position, without the nail or screw providing an electrical connection between the electrode layers 12A, 14A.

It will be apparent from the above that the structures described are sufficiently thin to be usable in confined spaces and can be constructed of flexible material to allow them to be wrapped around structures such as pipes, or laid underneath the sources of potential leaks, such as tanks. Other possible applications provide remote sensing of leaks or bursts in domestic lofts and attics, under floors, around or under pipe runs and tanks. The device can be used near machinery such as washing machines, and could be connected to remote alarm systems, or to the control circuits of the machine in order to alert the machine to the leak. The device can also be used as an incontinence alarm for use in bed, or in clothing, such as the clothing of a wheelchair occupant. It is envisaged that the sensor could be incorporated in a surgical dressing to detect haemorrhaging of a wound.

The sensor could also be used to form a screen around sensitive machinery such as computers. By earthing one of the sheet electrodes, the cage effect produced will prevent radiation entering or leaving the computer, to prevent the operation of the computer being illicitly monitored.

Claims

1. A liquid sensor comprising first and second electrodes attached respectively to first and second layers of insulating material, the insulating layers being attached together to form a unitary structure in which the electrodes are electrically insulated one from the other, the insulating layers being attached together at at least one attachment position, there being at least one other position in which the insulating layers are not attached.

2 A sensor according to claim 1, wherein the first electrode has a first edge beyond the second electrode projects and is exposed.

3. A sensor according to claim 1 or 2, wherein the first electrode and the insulating layers each have at least one gap formed therein, the gaps being aligned to allow electrically conducting fluid to form an electrical connection from the first electrode through the gaps to the second electrode.

4. A sensor according to claim 3, wherein the first electrode is formed by a sheet of electrically conducting material, the or each gap being formed between adjacent regions of the sheet.

5. A sensor according to claim 3 or 4, wherein at least one of the insulating layers is formed by a sheet of electrically insulating material, the or each gap being formed between adjacent regions of the sheet.

6. A sensor according to any of claims 3 to 5, wherein at least one of the gaps is a perforation wholly bounded by the electrode or insulating layer in which the perforation is formed.

7. A sensor according to any of claims 3 to 6, in which the first electrode has a second plurality of gaps formed therein in addition to the gap or gaps as aforesaid, the second gaps allowing the sensor to be penetrated by a fixing nail located in one of the second gaps, without an electrical connecting being established between the electrodes.

8. A sensor according to any of claims 3 to 7, wherein the second electrode has gaps formed therein and aligned with the gaps in the insulating layers and the first electrode, the gaps in the second electrode being closed by a material through which liquid in the gaps is visible.

9. A sensor according to claim 8, wherein the closing material is a sheet which extends across substantially the whole of the second electrode and the gaps formed therein.

10. A sensor according to any preceding claim, wherein the insulating layers are attached along one or more attachment lines which run along the edge or edges of the electrodes, whereby liquid is prevented from passing inbetween the sheets.

11. A sensor according to any preceding claim, wherein the second electrode is a sheet of electrically conducting material.

12. A sensor according to any preceding claim, wherein at least one of the insulating layers or the first electrode is formed by at least one elongate strip of material laid side by side in the sensor to form gaps between adjacent strips.

13. A sensor according to any preceding claim, further comprising a pad laid against the first electrode and of a material which allows liquid to penetrate the pad, and wherein the first electrode and the insulating layers are sufficiently thin to allow an electrically conductive path to be formed between the electrodes through liquid within the pad.

14. A sensor according to claim 9, wherein the pad is lightly bonded to the first electrode to allow the pad to be removed and replaced.

15. A sensor according to any of the preceding claims, wherein the first electrode is formed from a metal foil.

16. A sensor according to any of the preceding claims, wherein the insulating layers are formed from sheets of plastics material.

17. A sensor according to claim 12, wherein the sheets are attached by welding.

18. A sensor according to any of the preceding claims, wherein the insulating layers and the electrodes are all flexible.

19. A liquid sensor substantially as described above with reference to the accompanying drawings.

20. Any novel subject matter or combination including novel subject matter herein disclosed, whether or not within the scope of or relating to the same invention as any of the preceding claims.