Design and implementation of a fibre-optic based residual chlorine monitor

The design and implementation of a prototype optical residual chlorine monitor (RCM) for the purpose of chlorine concentration measurement in continuous flow is reported. A precision of 0.1 mg/l measured as OCl- is reported and the upper limit of the measurement range can be extended from 1 to 20 mg/l with a change in concentration of an added reagent. A background reference to allow for the presence of other interfering species is obtained by taking the ratio of the signals measured before and after chlorine is removed from the sample, with the addition of sodium sulphite. This compensates for any lamp and electronic-circuit drifts. The prototype has been tested over a wider range of waters and long periods of up to 24 h.