Moisture sensor

The moisture sensor comprises a block of absorbent material, such as wood. Two opposed faces are coated with conductive material, such as paint and two conductors of a cable are connected to the conductive material. The entire element is encapsulated within a sleeve of electrically insulating material, leaving one end face of the block of moisture absorbent material exposed. A plurality of apertures or perforations are provided in the sleeve exposing at least part of the moisture absorbent material. The sensor is adapted to be exposed to an environment to sense the moisture level of that environment.

BACKGROUND OF THE INVENTION 
The present invention relates to a moisture sensor and more particularly to 
a moisture sensor adapted to be used to sense the moisture content within 
wood, although the sensor of the invention may find other applications. 
It is to be appreciated that the performance and durability of wood is 
dependent upon its moisture content. It is thus desirable to be able to 
measure the moisture content of wood. 
Many moisture meters have been proposed previously which can measure the 
moisture content of wood, but most of these meters rely on inserting two 
prongs or electrodes into the wood, and then measuring the electrical 
resistance between the prongs or electrodes. 
Such a technique is very prone to error, since the electrical resistance 
between the prongs depends upon the degree of insertion into the wood, the 
nature of any paint provided on the exterior of the wood, and many other 
similar factors. Also such moisture meters can only be used to determine 
the moisture of wood where access can be gained to the surface of the 
wood. 
The present invention seeks to provide an improved moisture sensor. 
SUMMARY OF THE INVENTION 
According to this invention there is provided a moisture, sensor, the 
sensor comprising an elongate element of moisture absorbent material 
presenting two opposed side faces, the two opposed side faces being 
provided with conductive material extending over a predetermined area 
thereof, there being a cable having two conductors which are connected 
respectively to the areas of electrically conductive material, the element 
being encapsulated within a sleeve of electrically insulating material, 
the sleeve being such that an end face of the sensor is exposed, there 
being a plurality of apertures or perforations in the sleeve exposing at 
least part of the element, the sensor being adapted to be exposed to an 
environment, the moisture level of which is to be sensed by the sensor. 
Preferably the apertures of perforations are provided at the ends of bores 
extending through the element. 
conveniently the bores extend through parts of the element which are not 
provided with said conductive material. 
Preferably the electrically conductive material is in the form of 
conductive paint, such as a silver paint. 
Advantageously the conductors of the cable are secured or bonded to the 
conductive material. 
Advantageously the absorbent material is wood. 
Conveniently the absorbent material has a size which is approximately 1.8 
mm by 1.8 mm by 8 mm. 
The sleeve may be a heat-shrunk sleeve of plastic material which covers the 
conductive material. 
The invention also relates to the use of a sensor as described above with a 
moisture meter or a moisture monitor, and the sensor may be interrogated 
by a microprocessor. The sensor may be interrogated over a telephone line 
or the like by a remote computer.

DETAILED DESCRIPTION OF THE INVENTION 
A moisture sensor in accordance with the invention has a sensor strip 1 
which is formed from an appropriate element of a moisture absorbent 
material, such as wood. The wood may be any appropriate wood, but is 
preferably a uniform close-grained wood, so that sensor strips for sensors 
in accordance with the invention may be substantially uniform. The strip 1 
is an elongate strip of square cross-section. The strips may be cut to 
have approximately the desired size and the strips may then be brought to 
an accurate size by dry sanding or abrading. This will also remove 
splinters. It has been found that the sensor strip 1 may suitably have a 
final size of 1.8 mm by 1.8 mm by 8 mm. Of course, this is one example of 
a size that may be used for embodiments of the invention, but other sizes 
may prove to be adequate, be they larger or smaller. 
It is to be understood that the moisture content of the wood strip must be 
monitored and controlled, so that accurate dimensions for the finished 
sensor may be obtained. Preferably the strip should have a uniform 
moisture content of between 6 and 9 percent. 
When the sensor strip 1 has been fabricated to the desired size, two 
opposed elongate faces 2 which may be termed "radial" faces, are painted 
with electrically conductive silver paint. Any appropriate silver paint 
may be used, Such as ALTA 1 electrically conductive paint (Catalogue 
number 1 SECP). Only one of the faces 2 coated with silver paint is 
visible in FIG. 2. Care is taken to remove silver paint from all the other 
faces, or preferably to ensure that none of the paint is applied to the 
other faces. A cable 3 having two conductive wires 4, 5, is taken and the 
end portions 6 of the wires are bared, and the bared wires are then placed 
in contact with the respective silver painted faces 2 of the sensor strip 
1, and clamped in position. The bared wires become bonded or adhered to 
the silver paint as it dries. For example approximately 6.5 mm of bare 
wire may be bonded to each silver painted face 2 of the sensor strip 1. 
Next, the sensor is encapsulated in a strong electrically insulating water 
impermeable material. Thus, for example, a sleeve 7 of heat shrinkable 
plastics material is mounted on the assembly thus fabricated, to cover the 
entire length of the sensor strip 1, and to cover part of the cable 3. The 
sleeve 7 is then heat-shrunk into position, to form an encapsulation 8. 
The sensor strip 1 is thus substantially encapsulated, whilst leaving an 
end face 9 of the sensor strip exposed. The rest of the sensor is 
substantially sealed. 
A plurality of apertures are formed in the encapsulation, aligned with the 
elongate faces of the sensor strip 1 which are not provided with the 
silver paint. In the embodiment illustrated in FIG. 4, five apertures 10 
are provided. The apertures are evenly spaced and each have a diameter of 
approximately 0.5 mm. The apertures, in this embodiment, do not simply 
expose the unpainted side faces of the sensor strip, but instead form the 
ends of bores which extend through the sensor strip and out of the other 
side of the encapsulation. Thus, various regions of, the sensor strip are 
exposed including the end face 9 and the interior of each of the bores 
defined by the small holes or apertures 10. 
The sensor may be trimmed to the desired length by cutting back the exposed 
face 9. 
The fabricated sensor element is then permitted to reach an equilibrium 
moisture condition. This may be achieved by leaving the sensor within a 
standard environment for a specific period of time. The sensor element may 
then be connected to a resistance meter or moisture meter to check that 
the sensor element reads within a predetermined tolerance band. 
It is to be appreciated that the electrical resistance of the sensor 
follows the equation: 
##EQU1## 
where 
R=electrical resistance ohms 
r=electrical resistivity, ohm/mm 
d=is the distance between the electrodes in mm 
A=is the area of each electrode in square mm. 
It is possible to calibrate the sensor with regard to moisture content, the 
resistivity of the wood of the sensor varying with the moisture content of 
the wood. 
A sensor element in accordance with the invention may be located 
permanently or semi-permanently in position in a piece of wood, the 
moisture content of which is to be monitored, with the cable 3 being 
accessible to a moisture meter 12. The sensor element 1 may thus be 
located in a position which ordinary would not be accessible. Thus the 
sensor of the present invention may be utilised with a moisture meter 
which can be releasably and reliably connected to the cable 3 with greater 
facility than the conventional moisture meter which involves the insertion 
of pins into the wood work. It is to be appreciated that the sensor of the 
invention may be located in position when a wooden structure is 
fabricated, and the cable 5 may come to a termination at a suitable place, 
thus meaning that decorations need not be damaged when the moisture level 
of wood is to be determined. 
Alternatively, the sensor may be connected to a device with which it forms 
a moisture monitor 14, such as a micro-processor, which can effectively 
take readings of moisture level by interrogating the sensor from 
time-to-time. The measured readings may be recorded, and an alarm may be 
activated, or some device operated (such as a ventilating fan) if the 
measured value exceeds a predetermined limit, or is outside an acceptable 
range. 
Such a micro-processor can be pre-programmed appropriately in accordance 
with the particular conditions under which the sensor is to be used. 
The sensor may be interrogated from a remote location, for example over 
telephone lines 16, by a central computer 18. 
It is to be appreciated that the heat-shrunk sleeve provides the sensor 
with protection against physical damage or contamination when it is 
inserted into position. It has been found that the encapsulation controls 
the response time of the sensor. Encapsulation of the sensor with the 
heat-shrunk sleeve makes possible a miniature sensor of robust 
construction capable of withstanding physical and thermal abuse without 
loss of performance. 
The provision of the heat-shrunk sleeve also serves to isolate or insulate 
the sensor element electrically from the material in which it is 
installed, thus ensuring that the resistivity readings that are taken are 
taken solely across the sensor element. This allows the sensor element to 
be used in any type of material without any need for recalibration. 
It is to be appreciated that the material forming the strip of the sensor 
is preferably wood, as described, but this need not be so, since any 
appropriate absorbent material with the requisite properties can be 
utilised. 
The surface area of the sensor strip 1 that is exposed to the environment 
that is to be messed can be controlled by the encapsulation established by 
the heat-shrunk sleeve 8. Whilst in the described embodiment the entire 
end face 9 of the strip is exposed, a lesser area may be exposed if 
desired. 
In alternative embodiments of the invention the bare wires may be adhered 
to the silver painted faces 2, when they have dried, using a cellulose 
acetate glue or other appropriate adhesive. 
It is to be appreciated that the sensor is very small, and thus the sensor 
may be located in very small places. Also the sensor, since it is 
encapsulated, may be handled easily. 
The provision of the apertures in the encapsulation and the associated 
bores permits the response time of the sensor to be initially selected. 
The bores serve the dual function of decreasing the mass of the sensor 
element, while increasing the surface area. Thus the greater the number of 
bores the faster is the response time. However, the provision of the 
apertures and the bores does not detract from the advantages of 
encapsulating the element. 
Whilst the invention has been described with reference to the use of the 
sensor primarily to sense the moisture level present in timber, the sensor 
may be used to sense the moisture level present in the atmosphere, in a 
confined space, or in materials other than timber. 
The features disclosed in the foregoing description, in the following 
claims and/or in the accompanying drawings may, both separately and in any 
combination thereof, be material for realising the invention in diverse 
forms thereof.