The Flowmaster Capacity Monitor (HydroLogic number 19020) measures the gallon and filter capability of the Evolution RO1000, Merlin Garden Pro, and Tall Boy Filter Systems. It features a 3/8-Inch Quick Connect fitting. Quick Connect fittings make installation easy and mess-free. Do you want to know when it is time to change your system's filter cartridges? This monitor measures the flow through your system, so that you can more accurately know when your filters have reached their maximum capacity. The HydroLogic Flowmaster Gallonage and filter capacity monitor fits standard 3/8-inch water lines. Helps to customize your indoor hydroponic or outdoor garden. Ensures continual peak filtration with more timely filter changes. Accurately tracks the number of gallons of filtered water used.