This invention relates to an improved method of and device for measuring the moisture content of a material.

It is well known that a measure of moisture in materials can be obtained by measuring the electrical properties of the material, namely electrical resistance, capacitance, radio frequency (rf) admittance or impedance, etc. In the case of resistance measurement usually two pins, used as electrodes, are pushed into materials like wood, plaster and other porous building materials or, indeed, materials such as those used for building boats like GRP laminate. A disadvantage of the last-noted arrangement is that pin holes are left behind after measurement.

Another type of instrument used for measuring moisture control of materials uses rf capacitance measurement. Usually an instrument of this type utilises two electrodes mounted on a flat surface. The electrodes have a radio frequency oscillator connected to them and the near-field capacitance is measured. Higher moisture contents showing higher capacitance readings and vice versa .

This technique has a number of disadvantages as follows: a) a relatively large area needs to be presented to the surface and to be in good contact with it for reliable measurements. Rough surfaces cause measurement problems; b) as the measurement is made between flat electrodes on the same plane any "skin" moisture close to the electrodes will have a predominant effect on the measurement. Thus, a laminate several millimetres thick with condensate or other water film on the face will show

an extremely high reading which is not a true indication of the total water content of the laminate.

It is an object of the invention to provide a novel method and apparatus for measuring moisture content which overcomes the above-noted problems.

According to the invention, there is provided a device for measuring the moisture content of a material, the device including a body having a first electrode and a second electrode at a position on said body which is spaced substantially from said first electrode, means for applying a radio-frequency voltage signal between said electrodes and means for measuring the free-field impedance between said electrodes, whereby the moisture content of a mass of material affording a surface can be measured by holding said body so that said first electrode is closely adjacent to or contacting said surface and the body extends away from said surface so that said second electrode is relatively remote therefrom, operating the device to apply a said radio frequency signal between said electrodes and measuring the free-field impedance between said electrodes.

An embodiment of the invention is described below by way of example with reference to the accompanying drawings, wherein:

FIGURE 1 is a diagrammatic side perspective view, in phantom, of a device embodying the invention;

FIGURE 2 is a graph showing, for comparison, the response of a known rf moisture measuring device and a device embodying the invention, in particular test conditions; and

FIGURE 3 shows in perspective, from two different aspects, a moisture measuring device embodying the invention.

Referring to the drawings, the device comprises an elongate body 10 defined by a plastics casing having mounted within it adjacent one end, a first electrode A and further having mounted within it, adjacent the flat lower wall of the casing, a second electrode B, which may, for example, take the form of a flat metal sheet or foil. The electrodes A and B are connected with circuitry (not shown) within the casing, for applying a radio frequency voltage between the electrodes A and B, the electrodes being otherwise electrically isolated from one another. ^" The circuitry within the casing is also arranged to drive a display 12 (such as a galvanometer or a LCD dis_play) and to provide _^ via said display, a visual indication of the rf impedance between the electrodes A and B.

When the instrument is switched ON, with the instrument spaced substantially from any electrically conductive material, the radio frequency field emanating from the front end of the instrument (i.e. the end at which electrode A is disposed) is normally very weakly coupled to the electrode B in the body of the instrument and no reading is obtained. To test for moisture in a mass of material, for example a mass of solid material having a flat surface, the front end of the instrument is placed against that surface with the longest dimension of the body 10 extending substantially normal to such surface so that the electrode B is spaced substantially from that surface. Where the mass of material providing such surface is a dry electrically insulating material, the electrode A is again very weakly coupled with electrode B so that again no reading is obtained with the device switched on.

However, when the device is placed, in a similar manner, against the surface of a mass of material which may in itself be non-conductive, but which has some moisture present within it, the water within the material acts to transmit the radio frequency signal allowing the moisture- containing material to act as an emitting aerial re- radiating the rf field to the electrode B in the lower half of the instrument, normally through the body 10. No part of the electrodes A or B or indeed any part of the electronic circuitry is in direct electrical contact with the material being tested. The "coupling" between the electrodes and the moisture-containing material is entirely made through radio frequency transmissions capable of passing through electrically insulating materials like plastic. This mode of measurement predominantly measures the mass of water in front of the instrument which in certain circumstances is a decided advantage.

Figure 2 shows a comparative reading of several layers of wet film interleaved with an insulation material such as a film of pvc. In Figure 2, the quantity plotted along the Y-axis is admittance (i.e. the reciprocal of impedance) , whilst the number of layers is plotted along the X-axis. It will be seen that using the conventional two electrode near-field measurement the first film shows a high reading but that subsequent further films do not add to the reading. In the case of the radio frequency free- field admittance measurement using the apparatus described with reference to Figure 1, the reading increases with every extra thickness of wet film. This makes the instrument much more useful for determining the total mass of water through a laminated material disregarding surface layers which may be damp from condensation or rainfall.

However, sometimes it is an advantage to be able to measure surface conditions predominantly. This can also be achieved with the same instrument, by using an additional clip-on transfer electrode as shown in Figure 3. The transfer electrode, indicated at 16 in Figure 3, when fitted, provides, at the front end of the device, below the rounded or bevelled front end of the casing proper, a second, similarly rounded or bevelled forwardly facing surface whereby, when the front end of the instrument fitted with the transfer electrode is held against a flat surface of material to be tested, the rounded or bevelled front end of the casing proper can make substantially line contact with said surface along a first line and the rounded or bevelled forwardly facing surface of the transfer electrode can make substantially line contact with said surface along a parallel line below the first line. The term "transfer electrode" is applied for convenience to the item 16 in its entirety, but it will be appreciated that it is unnecessary for the item 16 to be electrically conductive in its entirety. Indeed, it is envisaged that the item 16 will comprise a plastics outer shell with electrically conductive foil extending over the surface which, when the item 16 is fitted lies immediately below the lower wall of casing 10, below the electrode B, such foil extending, within the shell at least to the location directly behind the peak or ridge of the bevelled front part of such shell which is to engage the surface to be tested. Thus, because of the close proximity between the foil and the electrode B and the relatively large area of overlap with electrode B, there is very close coupling between the foil and the electrode B, so that the foil effectively forms an extension of the electrode B terminating at the front of the device. It will be understood that the arrangement is such that, with the transfer electrode 16 fitted, there is a sufficient

dielectric gap, between the electrode A and the conductive foil at the front end of the shell, for the rf coupling between such foil and the electrode A to be negligible when the front ends of the device and electrode 16 are not engaged with the surface of a body of damp or otherwise electrically conductive material. Thus with the transfer electrode fitted, the base of the instrument is now electrically rf coupled to the active area adjacent to the normal working tip of the instrument. The double-ended instrument now acts as a near-field measurement device which is inherently more sensitive to water layers close to the sensing elements.

The preferred embodiment of the device thus comprises a single instrument which can normally measure moisture deep within a structure using the rf free-field technique but which, by fitting the transfer electrode described above, can also be used to make near-field surface measurements.

Whilst, as indicated, the devices described with reference to the drawings measure the free-field impedance between the electrodes, in general it will be preferably for the display 12 to be calibrated in terms of moisture content.