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Abstract

The feasibility and the design of a fibre-optic based sensor for measurement of the concentration of molecular species dissolved in aqueous solutions has been investigated. The use of optical fibres as an alternative approach to the use of conventional optical procedures has been demonstrated to show significant advantages in terms of cost and flexibility in design as well as sensitivity and reliability in the measurement.

Apart from the first two chapters, which review the field and outline the theoretical background to the sensor operation, the work reported is experimentally based with an appropriate theoretical foundation, culminating in the construction of a fibre-optic sensor for the detection of dissolved ammonia in water. The sensor is based upon an absorption technique in which the change in the optical spectrum with the concentration of ammonia dissolved in water is monitored. This technique is used with a small volume of indicator dye mixed with an ammonia buffer solution, which is injected into the sensor chamber. This dye is linked into a optical fibre and the optical absorptivity at an absorption peak of the indicator dye is determined via the transmission through the optical fibre.

The construction of the sensor is described, it consisting of two major parts: (a) the production of the sensor head, and (b) the interfacing of this head into a computer- based controller. The sensor head consists of three major components, a mixture of the indicator dye with a buffer solution, the optical fibre and the membrane mechanism. The sensing material includes the pH-indicating dye phenol red, ammonium chloride (NH4CI) as a buffer and distilled water. This mixture was injected into the small sensor chamber which was sealed and surrounded by an ion selective membrane for ammonia detection, then interfaced into a fibre optic bundle. This enables the sensing material to be in direct contact with the end of the fibres, and hence penetrated by the ammonia samples.

The operation of the sensor is thus that ammonia gas, to be detected, dissolved in to the water contained in the samples, changes the pH of the dye. This induces a change in the optical absorption of the dye mixture because of the change in the colour of the phenol red. This change is probed, via the fibre optic, in terms of the light propagating along the transmitting fibres. The resultant decrease in transmitted intensity can the be related to ammonia gas concentration. In the case of pH, an accuracy of ±0.05 pH units was achieved and for the ammonia monitor ±100gg/l ammonia concentration was obtained with a reproducibility in the order of ±3%.

Finally the sensor is evaluated for its overall suitability for ammonia detection. The best sensor constructed can detect dissolved ammonia gas concentration over the range 0 to 2%, to an accuracy and lower detection limit potential in the parts per million region. The sensor has an extremely rapid response, less than 0.2 seconds

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TK Electrical engineering. Electronics Nuclear engineering
Departments:	School of Science & Technology > Engineering > Electrical & Electronic Engineering School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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