The Flowmaster measures how many gallons of filtered water have gone through the system and alerts you when it fts time to change the filter. You can program in the filter capacity and when you reach the target number of gallons, the Flowmaster beeps. This is important for knowing when to change the pre-filter for the Merlin-Garden Pro. This unit also ensures your system runs at its peak efficiency for removing contaminants. Available in 3/8 in. quick connect for the Merlin-Garden Pro & Tall Boy and in 1/4 in. for the Small Boy.