Alistair MacLean

Hydrogel/fiber optic sensor for distributed measurement of humidity and pH value

The combination of chemically sensitive, swellable polymer materials with novel optical fiber cable designs to transduce the swelling activity into microbend loss enables a simple yet powerful sensor to be produced. Interrogating such cables with standard optical time domain reflectoctrometry (OTDR) instruments allows particular chemicals of interest to be detected and located along a cable which may extend to several kilometers. We report here on a sensor cable which uses a water swellable material, a hydrogel, to detect positions of water ingress, relative humidity level or pH value. In direct water ingress tests, wet sensor lengths as small as 5 cm in several hundreds of meters have been detected using conventional OTDRs. Following a review of the sensor design, we present the results of an investigation of the mechanical interaction between the hydrogel polymer and the optical fiber within the sensor. The behavior of the sensor is then characterized within environments of different relative humidity levels from 70 percent to 100 percent at temperatures ranging from 0 to 60 degrees C. The sensor was initially designed for applications within civil engineering but can be applied to a much broader range of measurement requirements, for example soil moisture measurement. We will report details on experimental observations on concrete cure within reinforcing tendon ducts and soil humidity measurements within different soil types.

Distributed fiber optic sensors for humidity and hydrocarbon detection

A novel distributed fiber optic sensor that incorporates liquid swellable polymers to transduce the swelling into a microbend loss is presented. Interrogation of the sensor using standard optical time domain reflectometry (OTDR) instruments provides the possibility of detecting target chemicals and fluids at any location along the sensor length. The location of multiple events along a sensor, which may extend to 4 km is readily achievable. In this paper we present an overview of the work conducted on the characterization of a distributed optical fiber water sensor. Following a discussion of the basic principles of the water sensor and the underlying technology we present a review of the significant developments achieved. Tests incorporating the sensor in civil engineering applications, which range from monitoring of concrete curing to leak detection in highways, are described. In addition to this, more recent developments to utilize the sensor technology to detect other fluids are discussed, in particular for the monitoring of pH changes and liquid hydrocarbons. We discuss some of the significant advantages in using this type of sensor construction and areas in which it can be practically used.

Remote gas analysis using fibre optic links and near infrared absorption

This paper describes the potential application of optical fibre addressed systems in remote gas spectroscopy. The paper will first describe the basic principles of the spectroscopic measurements and the reasons why such measurements find applications complementing more established mid infrared and electrochemically-based systems. We shall describe some practical field trials and the results obtained there from. Finally we shall discuss the potential offered by new approaches in laser design and system architectures to enhance the range of addressable species and the sensitivity which these remote detection systems may achieve.

Distributed fiber optic sensor for liquid hydrocarbon detection

A distributed fiber optic sensor for the detection and location of hydrocarbon fuel spills is presented. The sensor is designed such that liquid swelling polymers transducer their swelling into a microbend force on an optical fiber when exposed to hydrocarbon fuels. Interrogation of the sensor using standard Optical Time Domain Reflectometry techniques provides the possibility of rapidly detecting and locating target hydrocarbon fuels and chemicals at multiple positions along the sensor length. Events can typically be located to a precision of 2 m over a 10 km sensor length. Sensor response time on exposure to the hydrocarbon fuel is within 30 seconds. A detailed explanation of the operational characteristics of the sensor and the underlying technology utilized in its operation is given. Experimental tests using prototype sensors to simultaneously detect three separate 50 centimeter-long events are described. The characteristics of the sensor response in a range of hydrocarbon fuels under varying environmental conditions were investigated. Some of the safety advantages in using the sensor and its practical implementation in continuous monitoring of pipelines or fuel containment vessels are discussed.

Distributed sensing for liquid leaks and spills

This paper describes a very simple generic approach to detecting leaks and spills using fibre optic sensing technologies. The system has been configured for distributed sensing and the sensitive fibre cable can detect one, or several wet section(s) one metre in length over distances extending to 10km with location accuracies of the order of 1 metre. Furthermore, by modifying the interface chemistry, the system will respond to either aqueous solutions or to hydrocarbon fluid with no cross talk from one to the other. The system also responds in time scales of seconds, and is reversible over - to date - indefinite number of cycles. The system can also be configured to respond to water or (some) hydrocarbon vapours, predominantly at high vapour pressures. Finally, much simplified shorter range versions of the sensors are currently being investigated with a view to detecting the occurrence of a leak or spill at any point in the path length of metres or tens thereof.

A distributed fibre optic sensor for hydrocarbon detection

We present a distributed fibre optic sensor that incorporates hydrocarbon-swellable polymers to produce a microbend loss. Detection of hydrocarbon fuels at multiple locations along a 2km sensor length is readily achievable using standard OTDR techniques.

Ultrasound detection of damage in complex carbon fibre/metal structures

We describe work carried out to monitor the structural health of a complex structure comprising both carbon fibre and metal components using ultrasound techniques. The work is designed to be used in a high performance car, but could find applications in other areas such as the aerospace industry. There are two different types of potential problem that need to be examined; the first is damage (e.g. holes, delaminations) to carbon fibre structure, and the second is damage to joints either between two carbon fibre components or between a carbon fibre component and a metallic one. The techniques used are based around the use of PZT transducers for both the generation and detection of ultrasonic Lamb waves. To date we have been carrying out experiments on mock-up samples, but are due to conduct tests on an actual vehicle. Lamb waves propagate in modes whose order is determined by the frequency thickness product. Their properties, such as phase and amplitude can be modified by the presence of damage, such as holes and delaminations. If we record the response of a healthy structure, we can then compare it with signals obtained on subsequent occasions to determine if any significant change has taken place. It is essential, however, to be able to differentiate between the effects of damage and those of environmental changes such as temperature. For this reason we have monitored the response of a sample at different temperatures both before and after drilling a hole in it to simulate damage. Depending on the positions of the transducers with respect to the damaged area, it is possible to detect either attenuation of the entire signal or changes in a specific portion of the signal produced by reflections. Results from these experiments will be presented at the conference. Signal processing techniques for separating damage from the effects of temperature will also be discussed. We also look at the deterioration of joints, which can either be epoxy bonded (carbon fibre to carbon fibre) or bolted together (carbon fibre to aluminium). In the case of the bonded structures we are looking at the effects of failure of the bond layer, whilst in the case of the bolted samples we are looking at loosening of the bolts. The debonding was simulated by joining together a flat plate of carbon fibre composite with an L-shaped carbon fibre piece using a couplant such as grease. Similar experiments were carried out using an aluminium anglebar bolted to the plate, with the bolts both tightened and loose. Signals of both the transmitted wave in the plate and the power coupled to the L piece were measured before and after debonding. This gives a more reliable measure of the change in power transfer between the two components as the joint/bond degrades. It was found that in order to get maximum coupling to the second component the frequency of the acoustic wave had to be altered. This is because in the bonding region the combined thickness of the components alters the modal propagation characteristics of the structure compared with those of the single component region.

Detection of structural damage in carbon fibre and metal structures using Lamb waves

The detection and location of holes in an isotropic aluminium plate using fibre Bragg grating rosettes to detect ultrasound Lamb waves is described. This is followed by a description of the anisotropic properties of a carbon fibre plate and their effect on hole detection. Finally, the issues involved in attempting to locate holes in an anisotropic samples are discussed and the possibility of achieving this assessed

Fiber optic system for distributed detection of liquid hydrocarbons

A fibre-optic system for the detection and location of hydrocarbon fuel spills with 2m accuracy over a total length of 10 km is presented. The sensor incorporates liquid-swelling polymers that transduce their swelling into a force on an optical fibre when activated. The standard Optical Time Domain Reflectometry (OTDR) technique is used to interrogate the sensor, which provides the possibility of locating target hydrocarbon fuels and chemicals at multiple positions along the sensor length. Response time of the sensor after exposure to the activating liquid is typically 30 seconds, dependent on the activating fuel. A brief explanation of the operational characteristics of the sensor and the underlying technology utilised in its operation is given. Swelling characteristics of the sensor polymer material in a range of hydrocarbon fuels and solvents is presented. Experimental test results using prototype sensors to detect simulated fuel spills at separate locations are then described. The ability to replace the polymer with materials that are sensitive to other liquids provides the possibility of sensing many other liquids using the same generic design.

Optical frequency domain reflectometry for interrogation of microbend-based optical fiber sensors

The use of low frequency (sub 20 MHz) incoherent optical frequency domain reflectometry (IOFDR) provides a low cost alternative to the conventional OTDR techniques often used in the interrogation of optical fiber microbend sensors. We have modeled the operation of the IOFDR and experimentally characterized the operation of this technique for monitoring distributed water sensors based on water-swellable polymers (hydrogels). We demonstrate that the IOFDR is capable of detecting and locating sections of increased loss in a graded index multimode optical fiber and discuss the fundamental limits on spatial resolution and range.