Because on-line analysers must be self sufficient for several weeks at a time, high reliability and very low maintenance are of major importance. Many methods and technologies may be used for water analysis: ion specific electrodes, classical colorimetry, reflection colorimetry, amperometry. These methods are convenient for laboratory measurement, but on site, suspended solids, turbidity, contamination and interference mean that results are not reliable. Moreover, these methods need reagents which have many disadvantages: high operating costs, poor stability and water pollution. Spectroscopy is the only method which gives high stability and high reliability for on-line measurements, and for more than 15 years, our customers have appreciated the effective low maintenance and..