WO1992002804A1

WO1992002804A1 - A humidity indicator with an encapsulated sensor

Info

Publication number: WO1992002804A1
Application number: PCT/DK1991/000217
Authority: WO
Inventors: Lars Olsen; Erik Frederiksen
Original assignee: Dansk Teknologisk Institut
Priority date: 1990-07-31
Filing date: 1991-07-26
Publication date: 1992-02-20
Also published as: DK167257B1; AU8322491A; DK182690A; DK182690D0

Abstract

In a humidity indicator, in which the sensor (4) is encapsulated in an envelope (1, 2), the novel feature is that the envelope (1, 2) has a very low permeability for water vapour, to such an extent that changes in the humidity in the surrounding air takes considerable time - from 14 days up to one year or more - to manifest themselves in the space (3) containing the humidity sensor (40). With this arrangement, a time-integrating effect is achieved, making the indicator suitable for long-time monitoring of e.g. building structures without being influenced by short-term humidity increases, such as caused by the weather or an occasional escape of steam. In the embodiment shown, the humidity indicator (4) is a piece of fine paper impregnated with both a hygroscopic salt and a substance changing colour when moistened, whereas a similar dummy sensor (6) is only impregnated with the latter. This makes it easier to ascertain whether the colour of the humidity sensor (4) proper has in fact changed.

Description

A HUMIDITY INDICATOR WITH AN ENCAPSULATED SENSOR

TECHNICAL FIELD

The present invention relates to a humidity indicator of the kind set forth in the preamble of claim 1.

BACKGROUND ART

In previously known humidity indicators of this kind, the envelope either comprises openings allowing the surrounding atmosphere to contact the humidity sensor directly, or the envelope has such a high permeability for water vapour, that humidity changes in the sur¬ rounding air require but a few minutes to manifest themselves in the space inside the envelope.

Recently, a need has arisen to monitor the humidity in various building structures in order to determine whether measures are to be taken to reduce their humidity. As the previously known humidity sensors mentioned react comparatively quickly to short-term humidity changes, such as caused by a passing rain storm or a temporary outlet of steam, they are not suitable for monitoring the humidity in structures of the kind referred to, as care must be taken before each observation that no such short-term humidity increase is occurring at the moment.

DISCLOSURE OF THE INVENTION

It is the object of the present invention to provide a humidity indicator of the kind referred to initially, with which it is possible to obtain meaningful obser- vations of the long-term variations in humidity in building structures and the like, and this object is achieved with a humidity indicator additionally exhibiting the feature set forth in the characterising clause of claim 1.

With this arrangement, the time taken for short-term humidity changes in the surrounding air to manifest themselves in the space surrounding the humidity sensor is so long, that the short-term phenomenon will normally have ended well in advance of any such manifestation. The actually observed values will thus reflect the long-time humidity variations in the structure being monitored.

Advantageous embodiments of the humidity indicator according to the present invention, the effects of which are explained in more detail in the following detailed portion of the present specification, are set forth in claims 2-5.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed specification, the present invention will be explained in more detail with reference to an exemplary embodiment of a humidity indicator according to the present invention shown in the drawing, in which

Figure 1 shows the indicator in a longitudinal sectional view taken along the line I-I in Figure 2,

Figure 2 is a longitudinal sectional view taken along the line II-II in Figure 1 and showing only the backing sheet with a humidity sensor and a dummy sensor in separate compartments, and

Figure 3 is a magnified view of a part of the front sheet, delimited by the dotted circle III in Figure 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The exemplary embodiment of an integrating humidity indicator according to the present invention shown in the drawing comprises an envelope consisting of two main parts, viz. a front sheet 1 consisting of one or a number of layers of a transparent or translucent material having a low, but finite permeability for water vapour, such as polyethylene, and a backing sheet 2, that may either consist of the same material as the front sheet 1 or of some other material with a low, but finite permeability for water vapour, including zero permeability.

The envelope 1, 2 comprises two compartments, viz. a sensor compartment 3 containing a humidity sensor 4, and a dummy compartment 5 containing a dummy sensor 6.

Both the humidity sensor 4 and the dummy sensor 6 consist of thin white paper, e.g. so-called cigarette paper, impregnated and/or covered with a substance, which is able to change colour on being moistened, such as e.g. cobalt thio-cyanate. In addition to this, the humidity sensor 4 alone is also impregnated with another substance, viz. a hygroscopic substance capable of absorbing water from the surrounding air and dissolving in this water, if the degree of relative humidity in the surrounding air is equal to or exceeds a threshold value, which is characteristic of the substance used in each case.

The following table shows examples of relatively harmless hygroscopic salts that may be used as the hygroscopic substance, with which the humidity sensor 4 is impregnated.

TABLE

Liquefies Ml H2O 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

The front sheet 1 and the backing sheet 2 are sealed to each other around two compartments 3 and 5, forming a pheripheral sealing edge 7 extending all the way around both the compartments, as well as a sealing bridge 8 extending between the two compartments and separating them in an air-tight and vapour-tight manner from each other.

The water-vapour permeability of the envelope 1, 2 may be adjusted by an appropiate choice of material for the envelope. If the backing sheet 2 is completely impermeable, then this choice will apply to the front sheet 1, which may consist of a single layer of polyethylene, or a number of such layers as indicated in Figure 3, showing three layers la, lb and lc. Experimentally, good results have been achieved with polyethylene film with a thickness of 0.012 mm. The diffusion resistance may be adjusted in the range 5-500 PAM, all depending on the time constant desired, which may lie in the range from 14 days to 1 year or more.

In use, the indicator is placed in a location, in which the relative humidity is to be monitored. Due to the relatively high diffusion resistance of the envelope, short-term variations, such as those caused by a short spell of high-humidity weather or by water being boiled in the vicinity of the indicator, will not substantially influence the degree of relative humidity in the compartments 3 and 5, whereas long- term increases in the relative humidity, that could be caused e.g. by insufficient ventilation or a leaky central-heating system, will eventually manifest themselves as increases in the relative humidity within the compartments 3 and 5. If the relative humidity in the compartment 3 rises to or above the threshold value for the hygroscopic substance, with which the humidity sensor 4 is impregnated, this sensor will change colour, thus signalling to an observer that a long-term increase in humidity has occurred.

When making observations at comparatively long intervals, the observer may forget the original "dry" colour of the humidity sensor 4, but as the dummy sensor 6 does not contain the hygroscopic substance, but only the colouring substance, the dummy sensor will so to speak constitute a memory of the original "dry" colour, with which the observed colour of the humidity sensor 4 may be compared.

If more exact observations are required, a number of indicators with different hygroscopic substances and hence different humidity threshold values may be used together at the same location, possibly integrated with each other with a common front sheet 1 and a common backing sheet 2 and a number of pairs of compartments corresponding to the number of threshold values desired.

To make it easier to observe the colour of the humidity sensor 4 and/or the dummy sensor 6, they may be provided with a contrast backing 9 in the form of black plastic film laminated onto the back of each sensor, or dark or black paint applied in a suitable manner, such as by spray, possibly in a pattern.

When not in use, the indicator should be stored in a dry atmosphere, such as in a completely air-tight container containing a drying agent, e.g. silica gel. An example of such a container could be a glass tube, at each end having a stopper, of which at least one contains a drying agent.

The exemplary embodiment of the humidity indicator according to the present invention as described above is suitable for use in cases, where a great number of points are to be monitored, and the cost of making observations is of minor importance. The principles of the present invention can, however, with the same effect be applied to humidity indicators, in which the change in the physical state of the sensor is sensed in other ways than visually, e.g. by measuring the electrical resistance. Even so, the use of both a proper sensor and a dummy sensor may be found useful, e.g. by letting the two sensors form two arms of a bridge network.

It should be borne in mind that the time constant being characteristic of the transmission of a change in humidity through the envelope 1, 2 does not solely depend on the actual diffusion resistance of the materials used, but also on the quantity and type of salt used, as well as the volume of the sensor compartment 3.

Claims

CLAIMS :

1. A humidity indicator of the kind comprising a) a substance (4) changing its physical state when the degree of humidity in its immediate surround¬ ings changes through a predetermined interval, and b) an envelope (1, 2), in which said substance is placed, c h a r a c t e r i z e d by the envelope's (1, 2) permeability for water vapour being so low, that changes in the degree of humidity outside of- the envelope do not manifest themselves in changes in the physical state of said substance until after a period of time of substantial duration.

2. An indicator according to claim 1, c h a ¬ r a c t e r i z e d in that said envelope consists of at least two parts, viz. a) a first part (1) having a certain finite permeabi¬ lity for water vapour, and b) a second part (2) being substantially impermeable for water vapour.

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 envelope or said first part respectively is constituted by a capsule, cover, bag or cap consisting of or compris¬ ing one or more layers of plastic membrane or paper.

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 a) that said first (1) and/or second (2) part is/are at least partly transparent or translucent to a degree making it possible to observe visually the change of physical state of said substance, and b) that said substance is of a kind changing its physical state in a visually observable manner.

5. An indicator according to any one or any of the claims 1-4 and of the kind comprising an extra substance (6), the observable physical characteri¬ stics of which are substantially identical to those of said first-mentioned substance in the latter's dry condition, c h a r a c t e r i z e d in that the extra substance (6) is placed in a compartment (5) separated (8) in an air-tight and vapour-tight manner from the space (3) containing said first-mentioned substance (4).