AN AIR-HUMIDITY INDICATOR AND A METHOD OF MAKING A HUMIDITY-SENSITIVE STRIP FOR SAME

Abstract: A humidity indicator comprises a transparent tube (1) containing a fibrous strip (3) impregnated with a hygroscopic salt and folded in its dry state into a zig-zag shape, the tube (1) being closed by a cap (6) until use. When used for measuring the relative humidity in a concrete floor (8) with a view to judging its suitability for glueing a floor covering (not shown) to it, a hole (9) is bored in the floor (8), after which the indicator, whose cap (6) has been removed immediately before, is inserted in the top of the hole (9), sealing lips (7a) ensuring an airtight and vapour-tight joint. When equilibrium between the space inside the tube (1) and the hole (9) has been established, and if the relative humidity in these spaces is equal to or greater than the liquefaction value for the hygroscopic substance in the strip (3), the water absorbed by the hygroscopic substance will soften the adhesive holding the paper fibres together, and under its own weight the strip will straighten out at least partly, indicating to an observer that the humidity is above some predetermined value.

Description: AN AIR-HUMIDITY INDICATOR AND A METHOD OF MAKING A HUMIDITY-SENSITIVE STRIP FOR SAME TECHNICAL FIELD The present invention relates to an air-humidity indicator of the kind set forth in the preamble of claim 1. BACKGROUND ART In indicators of this kind, it is known to utilize the ability of the hygroscopic substance to absorb water from the air to obtain various visible effects, such as a change of colour of a carrier substance or a change in electrical conductivity. These effects are not always convenient to use, especially when measuring the relative humidity at a great number of points in a building structure, such as a concrete floor, where it is necessary to ascertain the degree of relative humidity in order to determine whether the material in question is suitable for receiving glue or cement used for securing a covering, such as a fixed carpet or linoleum, to the structure. Thus, the relatively large number of indicators necessary will often make the process of examining the relative humidity mentioned prohibitively costly, and in other cases difficulties may arise in interpreting the visual effects observed, espcecially when using colour changes. DISCLOSURE OF THE INVENTION It is the object of the present invention to provide an air-humidity indicator of the kind referred to initially, that may be produced at a very low cost, while the visual effects are easy to observe and interpret, and this object is achieved with an air-humidity indicator of the kind referred to above, which according to the present invention is characterised by the features set forth in the characterizing clause of claim 1. With this arrangement, the moisture absorbed from the air by the hygroscopic substance will weaken the fold or folds, so that the constantly acting force will straighten or unfold the strip in an easily observable manner. Advantageous embodiments of the indicator according to the present invention, the effects of which are explained in the following detailed portion of the present specification, are set forth in claims 2-9. The present invention also relates to a method of producing a humidity-sensitive strip for use in an indicator according to the present invention, and this method is characterised by the steps set forth in the characterizing clause of claim 10. BRIEF DESCRIPTION OF THE DRAWINGS In the following detailed specification, the present invention will be explained in more detail with reference to the exemplary embodiment of a humidity indicator shown in the drawing, in which Figure 1 is a longitudinal sectional view through the indicator in its state before use, carrying a protective cap, Figure 2 is a cross-sectional view taken along the line II-II in Figure 1, and Figure 3 is a longitudinal sectional view of the indicator in use for determining the relative humidity in a concrete floor, said section being taken along a plane at right angles to the sectional plane of Figure 1. DESCRIPTION OF THE PREFERRED EMBODIMENT The humidity indicator shown in the drawing consists of a tube 1 made of transparent material, such as acrylic resin or the like. The tube 1 is open at one end, whilst the other end is closed in an air-tight and vapour-tight manner by an end wall 2, the latter preferably being integral with the tube 1. A paper strip 3 is secured to the end wall 2, in the example shown by being inserted through a slit in the latter and cemented to same by means of suitable cement or glue 4. The paper strip 3 is folded in zig-zag fashion, thus exhibiting a number of transverse folding lines 5. The paper strip 3 consists of a type of paper, in which the fibres adhere to each other *>y means of an adhesive that may be dissolved or at least softened by water, normal fine tissue paper having proved suitable. Before being inserted in the tube 1 in the manner shown in Figure 1, the paper strip 3 has been treated in the following manner: - first, the paper strip or the material, from which it is made, is impregnated with a solution of a hygro¬ scopic substance capable of absorbing water from the air having at least a predetermined degree of humidity, after which the paper is dried before the impregnating solution has dissolved the adhesive holding the fibres together, the paper thus having been impregnated and dried is folded in zig-zag fashion in the manner indicated in Figure 1, the paper being cut into strips at some suitable time before, during or after the above impregnating, drying and folding steps. As soon as the impregnated, dried and zig-zag folded paper strip 3 has been placed in the tube 1 in the position shown in Figure 1 and secured to the end wall 2, the open end of the tube 1 is closed in an air-tight and vapour-tight manner by means of a cap 6 cooperating sealingly with an annular sealing flange 7 made of resilient rubber or plastics material, that extends around the open end of the tube 1, said sealing flange having a peripheral sealing lip 7a. When the humidity indicator is to be used for measuring the humidity in e.g. a concrete floor 8, of which but a tiny part is shown in Figure 3, a hole 9 is drilled into the concrete floor 8, after which the cap 6 is removed from the sealing flange 7 and the tube 1 is placed in continuation of the hole 9 as shown in Figure 3 with the sealing flange 7 with its sealing lip 7a making an air-tight and vapour-tight joint between the tube 1 and the concrete floor 8. After a short period of time, a humidity equilibrium will be established between the space in the hole 9 and the space in the tube 1, and if the degree of humidity is sufficient to liquefy the hygroscopic substance, with which the paper strip 3 has been impregnated, water will be absorbed from the atmosphere inside the tube 1 and soften or dissolve the adhesive holding the fibres of the paper strip 3 together, thus reducing the bending strength of the paper at the folding lines 5 to such an extent, that the mere weight of the paper strip and of the water having been absorbed will cause the paper strip 3 to unfold at least partly as shown in Figure 3, thus clearly indicating to an observer that the humidity in the region of the concrete floor around the hole 9 is equal to or exceeds the liquefaction threshold for the hygroscopic substance being used. In cases where the test hole 9 is more or less horizontal instead of vertical as shown in Figure 3, the inherent elasticity of the paper fibres has proved sufficient to effect at least a partial straightening of the strip 3 when influenced by the absorbed moisture, thus providing the requisite visible effect. Thus, the constantly acting unfolding force may be the force of gravity, the inherent elasticity of the paper in the folding lines 5, or a force exerted by some external means (not shown). At shown in Figure 3, the paper strip 3 is tapered along straight-line edges to an end point 10. With this shape of the paper strip 3, the ratio between the length of each folding line 5 and the weight of the paper below it will be substantially constant, so that the "opening angle" will be substantially the same at all folding lines. As mentioned above, fine tissue paper has proved suitable for use in making the paper strip 3, but it could also be possible to use other fibrous web materials, such as so-called cigarette paper, non- woven fabric or the like, two essential conditions being that the adhesive holding the fibres together may be dissolved or softened by water, and that the material is sufficiently thin to unfold at least partly, when said adhesive is being dissolved or softened by moisture having been absorbed by the hygroscopic substance. The following table shows examples of relatively harmless hygroscopic salts that may be used as the hygroscopic substance, with which the paper strip 3 is impregnated. TABLE Liquefies Ml H 2 0 at rel. supplied to hum. liquefy (20°C) 100 g salt (0-20°C) .Anhydrous copper chloride 68.4% 142 Dihydrate of copper chloride 68.4% 90.6 Sodium chloride 75.5% 280 .Ammonium sulphate 81.0% 142 Potassium chloride 86.0% 288 Potassium nitrate 93.2% 752 Potassium sulphate 97.1% 833 If the humidity indicator made ready for use as shown in Figure 1 is to be stored for a considerable length of time before use, a cartridge or bag containing a drying agent, such as silica gel, may be placed inside the tube 1 before closing it with a cap 6. .Another possibility is to place the whole humidity indicator, either singly or in a small or large number, in an air-tight and vapour-tight container also containing a suitably dimensioned portion of such drying agent. In the first-mentioned case, in which a drying agent is placed inside the tube 1, care should be taken to avoid the drying agent contacting the paper strip 3; this could suitably be achieved by securing the cartridge or bag containing the drying agent to the inside of the cap 6. The paper strip 3 may have any suitable size, but is preferably about 1.5 mm wide, narrowing to zero at the end point 10, in which case it is preferably about 20 mm long. If so, the total length of the tube 1 with cap 6 would be about 13 mm and the insertion diameter of the sealing flange 7 about 5 mm, thus requiring a test hole 9 of only 5 mm in diameter.

Claims: CLAIMS : 1. An air-humidity indicator of the kind, in which the effect is based on a hygroscopic substance, which when acted upon by air with at least a predetermined degree of humidity absorbs water from the air and is dissolved in said water, c h a r a c ¬ t e r i z e d in that said hygroscopic substance is carried by a strip (3) of fibrous web material, e.g. paper, in which the fibres adhere to each other by means of an adhesive, that may be dissolved or softened by water, said strip (3) being secured (4) solely at one end or edge and comprising at least one sharp fold (5) between said end or edge and the opposite, free end or edge (10), said fold or folds (5) at least in the operative position of the indicator being acted upon by a force acting constantly in the unfolding direction. 2. An indicator according to claim 1, c h a - r a c t e r i z e d in that the fibrous material is fine tissue paper, cigarette paper or the like. 3. An indicator according to claim 1 or 2, c h a r a c t e r i z e d in that said hygroscopic substance is selected from the group of salts compris- ing anhydrous copper chloride, dihydrate of copper chloride, sodium chloride, ammonium sulphate, potassium chloride, potassium nitrate and potassium sulphate. 4. An indicator according to any one or any of the claims 1-3, c h a r a c t e r i z e d in that the strip (3) with one of its ends or edges is secured to the inside of an end wall (2) or side wall of an at least partly transparent container (1), the length or width of which is of the same order of magnitude as the length or width respectively of the strip in the unfolded state of the latter. 5. .An indicator according to claim 4, c h a ¬ r a c t e r i z e d in that said container (1) is formed as a tube closed at one end, in which one end of the strip (3) is secured to the inside of the closed end (2) of the tube. 6. An indicator according to claim 5, c h a ¬ r a c t e r i z e d in that said strip (3) is tapered, being secured at its widest end. 7. An indicator according to claim 6, c h a ¬ r a c t e r i z e d in that the strip (3) tapers to a point at its free end (10). 8. An indicator according to any one or any of the claims 5-7, c h a r a c t e r i z e d by an annular gasket or seal (7) placed on the outside of the tube at or adjacent to the letter's open end, said gasket or seal having such a shape, that when the tube is inserted in a circular opening (9) in e.g. a concrete floor (8), it seals the tube against the wall of the opening and/or the external surface of said floor (8). 9. An indicator according to any one or any of the claims 1-8, c h a r a c t e r i z e d in that it is adapted to be sealed in an airtight manner from the surroundings, either by the container (1) being provided with a removable, moisture-proof seal (6), or by the container (1) being placed in a moisture-proof bag, package or container, which may be opened without damaging the humidity indicator, and that may contain a drying agent, e.g. silica gel. 10. A method of producing a humidity-sensitive strip (3) for use in an indicator according to any one or any of the claims 1-9, c h a r a c t e r i z e d by the following steps a-c: a) fibrous web material, e.g. paper, of a kind in which the fibres adhere to each other by means of an adhesive, that may be dissolved or softened by water, is immersed in a solution of a hygroscopic substance of the kind, which when acted upon in its dry state by air with at least a predetermined degree of humidity absorbs water from the air and is dissolved in said water, said immersion being carried out for a length of time sufficient for said solution to impregnate said fibrous material but insufficient for said solution to dissolve said adhesive to a substantial extent, after which the fibrous material is dried, and b) the impregnated and dried fibrous material is folded along at least one folding line, whereas c) the fibrous material is cut into strip form at any time before, during or after the steps set forth in items a and b above.

Inventors: FREDERIKSEN ERIK, OLSEN LARS, FREDERIKSEN, ERIK, OLSEN, LARS
Applicants: DANSK TEKNOLOGISK INST, DANSK TEKNOLOGISK INSTITUT
Application year: 1991
Main classification: G01N19
IPC codes: G01N19