User selectable features and innovative design sets HF Scientific's CLX Online Residual Oxident and Chlorine Monitor apart from the competition. Cycle times that are adjustable from 110 seconds to 10 minutes, 4-20mA and Modbus outputs that can control feed pumps and user settable alarms contribute to low operating and maintenance costs. A new flow-through design maintains cuvette cleanliness, allowing for viewing of optical operations. The strong, shatterproof case that opens to facilitate reagent replacement also allows easy access to all service functions which reduces maintenance time. Unattended operation and low chemical usage equates to low operating cost.