An online analyzer measures water quality continuously, straight from the pipe, tank or channel. Its sensors read parameters such as pH, dissolved oxygen (DO), TDS, chlorine, turbidity and TSS; the controller shows them on a display and sends a 4–20 mA or RS485 signal to a PLC, PC or automatic dosing system.

For an ETP, that means a problem shows up the moment it happens — not at the next lab test — and the dosing system can correct it on its own.