Automatic and accurate control of chlorine in potable and waste water is more important than ever due to health and regulatory concerns. Poor control can lead to failure of biomonitoring tests leading to an even more costly “Toxicity Reduction Evaluation.” The 555 is a cost-effective way to upgrade chlorination and dechlorination processes in an easy to use, rugged 1/4 DIN size package. The 555 can be easily set up to handle flow proportional control, residual control, compound loop control with lag times, and dechlorination with sulfur dioxide.