Development of fibre-optic based techniques for measurement of aqueous chemical species

The feasibility and design of a fibre-optic based system for measurement of the concentration of molecular species dissolved in aqueous solutions has been researched. The use of optical fibres as an alternative approach to the use of conventional optical procedures has been demonstrated to have significant advantages in terms of cost and flexibility in design as well as sensitivity and reliability in the measurement. In the particular embodiments investigated, absorption of light in the visible and the ultraviolet part of the electromagnetic spectrum has been used to quantify both the hydronium ion (H30+) and hypochlorite ion (OC1) concentration in a range of natural waters and effluents. This was carried out by monitoring the variation of the light intensity at the sensing wavelength caused by the interaction of light with the molecules. Two different approaches were utilised, the first of which was based on the measurement of the absorption of light by a dye indicator whose concentration is chemically affected by the the species and this was used to monitor the concentration of the hydronium ion (indirect method). In the second approach, the variation of the light intensity caused by the absorption of the chemical species itself was monitored (direct method). The latter method was applied to monitor the concentration of chlorine in the form of hypochlorite ion in various types of water sources. Theoretical analysis of the indirect method was carried out and a simulation of the response of the sensor was provided with emphasis on the computation of the pH value in the biomedical range. The implementation of the sensor was based on the use of low cost optoelectronics and enhanced signal processing schemes. In this application, the reference signal, used to compensate for chemical and electrical interference, was obtained in two fundamentally different ways. In the one implementation of the pH sensor, an external light source was used while in another, the fluorescence of a ruby crystal was used to generate the reference signal. The latter scheme has the distinct advantage of using the same light source for sensing and referencing. Performance of the sensor was analysed in terms of noise and sensitivity. In the direct method, the monitoring of the chlorine as hypochlorite ion was carried out by measurement of light absorption in the ultraviolet part of the spectrum. Spectral characteristics were compiled to provide a basis for the design and implementation of an on-line residual chlorine monitor according to the water industry standards. Detailed evaluation of the sensor performance was carried out and analysis of the engineering problems and their solutions have been presented together with the performance of the sensor in long term operation. In the case of pH, an accuracy of ±0.05 pH units was achieved and for the residual chlorine monitor ±100μg/l chlorine concentration was obtained with a reproducibility in the order of ±3%