Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1995-12-13
1998-04-28
Westin, Edward P.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
385 13, 25022725, G01M 304, G02B 644
Patent
active
057447942
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to a detection system for use in detecting the presence of a particular or target measurand. The system has particular utility in the detection of target measurands which are transported by aqueous means. One example is the ingress of aqueous measurands into the interior of post-tensioned reinforced concrete sections.
Many large structures, such as road bridges, are constructed using post-tensioned reinforced concrete sections. This construction technique requires that steel tendons be inserted through ducts which run through the length of a concrete section. These tendons are tensioned in order to apply a compressive load to the concrete. The remaining area within the ducts is filled with cement-based grout which forms a seal around the tendons intended to prevent the ingress of moisture and de-icing salts which would otherwise promote corrosion of the steel, thus weakening the structure. However, in structures such as road bridges, settlement and the vibrations produced by traffic tend to crack the concrete and the grout, allowing water and other substances (e.g. chlorine) to reach the tendons, which are then subject to corrosion. Replacement of such corroded tendons is a difficult, and very expensive task.
Another example where detection of aqueous measurands is desirable is within electrical or optical fibre cable duct systems where there may be a danger of corrosion penetrating the insulation material and giving rise to an electrical short circuit fault, or to loss of optical transmission. Aqueous measurands may also be measured in situations of loss thereof from a container system (e.g. loss of fluid from a pipeline) or as a measure of quality of a fluid in a pipeline. However, the measurand need not be borne directly or indirectly by aqueous means, for example it may be irradiation borne.
It is an object of the present invention to provide a detection system which is capable of detecting the presence of such measurands, and thus allow repairs or other control measures to be carried out.
SUMMARY OF THE INVENTION
According to the present invention there is provided a detecting system for use in detecting the presence of a target measurand, said system comprising:
a fibre-optic probe assembly incorporating an optical fibre which is subject to micro bending at intervals along its length, the probe assembly comprising a rigid containment structure which is filled with the length of optical fibre and a body of material which is subject to a volumetric change on exposure to said target measurand; and
a sensor assembly coupled to the probe assembly, the sensor assembly having optical signal transmitting and receiving means arranged to identify optical fibre microbend changes arising, in use, from forces imposed locally on the fibre by the interaction of the rigid containment structure and volumetric changes in the body of material, characterised in that said body of material comprises a hydrogel based polymer.
The system may be used simply to detect the presence of a target measurand at an unspecified location along the length of the optical fibre, though it is preferred that said signal transmitting and receiving means is also capable of detecting the particular location on the optical fibre where the signal carrying property of the optical fibre has changed and thus also detecting the particular location of the affected portion of the body of material (which is preferably elongate).
In a modification the system may have more than one probe assembly in which case the sensor assembly is provided with a logic function output circuit to decipher the effects of different target measurands on the different hydrogel based polymers of the different probe assemblies.
The body of material may be extended to form a continuous rod or may be deposited as a coating on a former. Preferably the thickness of the coating is less than 50 microns.
The body of material may expand on exposure to one target measurand, and may contract on exposure to another target measurand
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Culshaw Brian
Gardiner Peter Thomas
Graham Neil Bette
Michie Walter Craig
Moran Christopher Raymond
Lee John R.
University of Strathclyde
Westin Edward P.
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