The HydroLogic TDS Pro Monitor (HydroLogic 28010) is a PPM monitor of total dissolved solids (TDS) with a 1/2-inch inlet and a 3/8-inch outlet for the Merlin Garden Pro Reverse Osmosis System. This monitor allows you an ROutine check of the parts per million (ppm) of particulates in the water used in your garden water filtration. The HydroLogic TDS monitor fits a standard 1/2-inch incoming water line and a standard 3/8-inch outgoing water line. Helps to customize your garden water filtration system. Ensures continual peak filtration with more timely filter changes. Accurately tracks the amount of particulates in your RO water.