GB2213596A

GB2213596A - Disposable humidity sensor

Info

Publication number: GB2213596A
Application number: GB8830400A
Authority: GB
Inventors: Fali Minocher Dadachanji
Original assignee: Protimeter PLC
Current assignee: GE Protimeter Ltd
Priority date: 1988-01-07
Filing date: 1988-12-30
Publication date: 1989-08-16
Also published as: GB8830400D0

Abstract

A disposable humidity sensor, for sensing the dampness or humidity of a wall or floor in a building, comprises a patch of flexible impervious film (A) having, on one side thereof a piece of moisture absorbent material (C) having a known moisture content/electrical conductivity relation, the patch having, around a border region thereof of said one side, an adhesive to secure it to the body of which the dampness is to be measured. After the patch has remained in place for a period sufficient to allow said piece of moisture absorbent material to reach humidity equilibrium with said body, the electrical conductance of said piece of moisture-absorbent materal is measured so that the dampness or humidity of the body under test determined. The electrical conductance of the material (C) may be measured by inserting through the impervious film (A), and into the piece of material (C), a pair of electrodes (H), at a predetermined spacing from one another, and measuring the electrical resistance between said electrodes. Alternatively the patch may itself provide an electrode assembly (B) connected with terminals (D) engageable by probes of an appropriately calibrated electrical measuring instrument. <IMAGE>

Description

DESCRIPTION OF INVENTION Title: "Disposable humidity sensor" THIS INVENTION relates to a device for sensing the dampness or humidity of a body or structure, for example, of the wall or floor of a building.

Dampnes in a structure can be measured by measuring the relative humidity of a small pocket of air bounded, on the one hand, by the surface of the structure, and, on the other hand, by an impervious enclosure, for example a waterproof tent of polythene or foil, or a box, held against said structure. Water evaporating from the structure into the small amount of air trapped in the tent or box will raise its relative humidity until it is in humidity equilibrium with the sample. The dampness of the sample can then be measured by measuring the relative humidity of this pocket of air.

This method is recommended for concrete floors -before floor-laying - and for walls - before painting. Devices available for this form of measurement are usually cumbersome and time-consuming to apply and expensive to make and use.

It is an object of the present invention to provide a humidity sensor for use in measuring the dampness of a structure or body by the above-noted method, which can be applied quickly and easily and which can be made so cheaply as to allow it simply to be disposed of after use.

According to one aspect of the invention, there is provided a humidity sensor comprising a patch of flexible impervious film having, on one side thereof, in a central area of the patch, a piece of moisture absorbent material having a known moisture content/electrical conductivity relation, the patch having, around a border region thereof, surrounding said central area, on said one side, an adhesive whereby the patch may be applied to a surface of the body of which the dampness is to be measured, and whereby, after the patch has remained in place on said body for a period sufficient to allow said piece of moisture absorbent material to reach humidity equilibrium with said body, the electrical conductance of said piece of moisture-absorbent material can be measured and hence the humidity of the piece of moisture absorbent material determined and thence the dampness or humidity of the body under test determined.

Preferably a semi-permeable membrane which will allow the passage of water vapour to said humidity sensitive membrane, but which will exclude liquid water and other contaminants, is extended across said central area over the moisture absorbent material, on said one side of the patch, said semi-permeable membrane being sealed at its periphery to said one side of said patch, leaving said adhesive border region exposed.

According to another aspect of the invention, there is provided a method of measuring the dampness or humidity of a body or structure comprising applying a humidity sensor according to the first-noted aspect to the surface of said body or structure to be fixed thereto by said adhesive, leaving the humidity sensor in place for long enough for said humidity sensing membrane to reach humidity equilibrium with said body or structure, and measuring the electrical conductance of said piece of moisture absorbent material, and thereby determining the humidity of said piece of material and thus the dampness or humidity of the material under test.

Embodiments of the invention are described below by way of example with reference to the accompanying drawings, wherein: - FIGURE 1 is a diagrammatic plan view of a humidity sensor embodying the invention, FIGURE 2 is a sectional view of the sensor of Figure 1 applied to the surface of a structure such as a wall, FIGURE 3 is a diagrammatic plan view of another form of humidity sensor embodying the invention, and FIGURE 4 is a sectional view of the sensor of Figure 3 applied to the surface of a structure.

Referring to Figures 1 and 2, the embodiment shown therein comprises a patch of a flexible impervious plastic film (A) on one side of which is deposited a pair of electrodes (B). The electrodes (B) are preferably flexible elements of negligible thickness, for example strips of thin foil or electrically conductive paint adhered to the film (A). A humidity-sensitive membrane (C), of which the electrical conductance varies with humidity, is applied to said one side of the film (A), over a limited area, extending over and bridging the electrodes (B).A pair of electrically conductive elements, connected with the electrodes (B), pass through the impervious membrane (A) to the other side thereof and provide, on said other side, contacts (D) to which an electrical measuring instrument may be applied for measurement of the electrical resistance or conductance of the humidity sensing membrane (C). Over the electrodes and the humidity-sensing membrane is applied a semi-permeable membrane (E) which will allow the passage of water vapour to the humidity-sensing element (C), but will exclude liquid water and most contaminants, e.g. dust.

This membrane (E) extends only over a central region of the patch, leaving exposed an annular peripheral or border region. The semi-permeable membrane (E) is sealed at its outer edge to the impervious membrane (A). A low-tack adhesive is applied to the aforesaid annular border region of patch (A) on said one side thereof to allow the sensing element to be simply fixed to the surface of the sample under test.

The patch, for storage and handling purposes prior to use, may have a release backing sheet, not shown, adhered thereto by the permanently tacky adhesive. When the sensor is to be used, the backing sheet is simply peeled off and the patch pressed onto the surface of the structure to be tested, as illustrated in Figure 2. For ease of illustration, in Figure 2, the film (A) is shown as forming a shallow dome over the electrodes and medium (C) and over the semi-permeable membrane (E). However, it will be appreciated that the thicknesses of the electrodes (B), membrane (C) and semi-permeable membrane (E) are actually negligible in relation to the other dimensions of the patch so that the latter lies substantially flat against a flat surface, or otherwise conforms closely to the surface to which it is applied.

In use, the sensor is held in place over the suspect area by the low-tack adhesive. Water evaporating from the structure will raise the humidity around the humiditysensing element until it is in humidity equilibrium with the structure. After a few hours an electronic hygrometer with a wandering lead, which is provided with contacts configured to fit exactly with the contacts (D) on the impervious membrane, allows a reading to be taken from the humiditysensing element. The instrument in question, although actually'measuring the electrical resistance or conductivity between contacts is, of course, calibrated directly in terms of humidity or dampness. A contact thermometer, e.g.

a thermistor or thermocouple, can also be placed against the sensing element to provide temperature compensation, if required.

In use, several sensing elements can be stuck onto say, a wall surface, at various points, and allowed to equilibriate. At the proper time an electronic hygrometer can be connected to each sensor in turn and the humidity measured at the respective point.

In Figures 3 and 4, parts corresponding to parts in Figures 1 and 2 have the same references.

Referring to Figures 3 and 4, an alternative embodiment comprises a patch of a flexible impervious plastic film (A) on one side of which is disposed a thin sliver (C) of timber or other material exhibiting a known and marked variation of electrical conductivity with dampness or humidity. Over the sliver of timber or the like is applied a semi-permeable membrane (E) which will allow the passage of water vapour to the sliver of timber or the like (C), but will exclude liquid water and most contaminants, e.g. dust. This extends only over a central region of the patch, leaving exposed an annular peripheral or border region. The semi-permeable membrane (E) is sealed at its outer edge to the impervious membrane (A).A low-tack adhesive is applied to the aforesaid annular border region of patch (A) on said one side thereof to allow the sensing element to be simply fixed to the surface of the sample under test.

The patch, for storage and handling purposes prior to use, may have a release backing sheet, not shown, adhered thereto by the permanently tacky adhesive. When the sensor is to be used, the backing sheet is simply peeled off and the patch pressed onto the surface of the structure to be tested, as illustrated in Figure 4.

In use, the sensor is held in place over the suspect area by the low-tack adhesive. Water evaporating from the structure will raise the humidity around the sliver (C) of timber or the like until it is in humidity equilibrium with the structure. After a few hours the probe (G) of a moisture measuring instrument of known type, (and which probe comprises two electrodes (H) in the form of sharp-pointed pins extending, parallel with each other, from a common holder (J) of insulating material, and which holder (J) maintains the electrodes at a predetermined spacing from one another), is applied to the patch so that the two pins (H) penetrate the impervious film (A) and are embedded in the sliver (C) of timber or the like.The electrical conductivity between the pins (H) is measured by the instrument (not shown) and the humidity of the sliver (C) and thus of the structure being tested, thereby determined. The instrument in question, although actually measuring the electrical resistance or conductivity between contacts is, of course, calibrated directly in terms of humidity or dampness. The sliver (C) of timber or the like is preferably so selected that the indication of moisture content or humidity given by the instrument, when the latter, as is preferable, is calibrated in terms of moisture content or humidity, will correspond with the humidity or dampness of the body or structure under test.

A contact thermometer, e.g. a thermistor or thermocouple, can also be placed against the sensing element to provide temperature compensation, if required.

In use, several sensors or patches can be stuck onto say, a wall surface, at various points, and allowed to equilibriate. At the proper time a probe (G) such as described can be engaged with each sensor or patch in turn and the humidity measured at the respective point.

Claims

1. A humidity sensor comprising a patch of flexible impervious film having, on one side thereof, in a central area of the patch, a piece of moisture absorbent material having a known moisture content/electrical conductivity relation, the patch having, around a border region thereof, surrounding said central area, on said one side, an adhesive whereby the patch may be applied to a surface of the body of which the dampness is to be measured, and whereby, after the patch has remained in place on said body for a period sufficient to allow said piece of moisture absorbent material to reach humidity equilibrium with said body, the electrical conductance of said piece of moisture-absorbent material can be measured and hence the humidity of the piece of moisture absorbent material determined and thence the dampness or humidity of the body under test determined.

2. A humidity sensor according to claim 1 wherein a semi-permeable membrane which will allow the passage of water vapour to said piece of moisture absorbent material, but which will exclude liquid water and other contaminants, is extended across said central area over the piece of moisture absorbent material, said semi-permeable membrane being sealed at its periphery to said one side of said patch, leaving said adhesive border region exposed.

3. A humidity sensor according to claim 1 wherein said patch of flexible impervious film has, on said one side thereof, in a central area of the patch, a pair of electrically conductive flexible members or tracks applied thereto and forming spaced-apart electrodes, said piece of moisture absorbent material being applied to said one side of the patch so as to bridge said electrodes, said electrodes being connected with respective conductors accessible to an electrical measuring instrument when the patch is applied to the surface of the body of which the dampness is to be measured.

4. A humidity sensor according to claim 3 wherein a semipermeable membrane which will allow the passage of water vapour to said humidity sensitive membrane, but which will exclude liquid water and other contaminants, is extended across said central area over the humidity sensitive membrane, and said electrodes, on said one side of the patch, said semi-permeable membrane being sealed at its periphery to said one side of said patch, leaving said adhesive border region exposed.

5. A method of measuring the dampness or humidity of a body or structure comprising applying a humidity sensor according to claim 1 or claim 2 to the surface of said body or structure to be fixed thereto by said adhesive, leaving the humidity sensor in place for long enough for said piece of moisture absorbing material to reach humidity equilibrium with said body or structure, inserting through said impervious film of said sensor, and into said piece of moisture-absorbing material, a pair of electrodes, at a predetermined spacing from one another, and measuring the electrical resistance between said electrodes as a measure of the dampness or humidity of said piece of material, and hence of said body or structure.

6. A method of measuring the dampness or humidity of a body or structure comprising applying a humidity sensor according to claim 3 or claim 4 to the surface of said body or structure to be fixed thereto by said adhesive, leaving the humidity sensor in place for long enough for said humidity sensing membrane to reach humidity equilibrium with said body or structure, and measuring the electrical resistance between said electrodes.

7. A humidity sensor substantially as hereinbefore described with reference to, and as shown in Figures 1 and 2 of the accompanying drawings.

8. A humidity sensor substantially as hereinbefore described with reference to, and as shown in Figures 3 and 4 of the accompanying drawings.

9. A method of measuring the dampness or humidity of a body or structure, substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

10. A method of measuring the dampness or humidity of a body or structure, substantially as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.

11. Any novel feature or combination of features described herein.