Measures unpurified water PPM going in to reverse osmosis system and purified water PPM coming out. Monitors the performance of your RO system and instantly displays values and allows you to switch back and forth between dirty and clean water TDS. The unit will indicate. when it?s time to change the RO membranes Pro model features larger, more water resistant display and shielded cables. ?” inlet and ?” outlet designed to be used with the Evolution-RO.