Merlin Flowmaster Gallon Monitor 

Never lose track of when to change the filters in your Merlin Garden Pro RO system! 

The Flowmaster Gallon Monitor keeps track of how much water the system has filtered, then lets you know when to change filters.

It's easy to use - simply program the water purification capacity of the Merlin's filter and it keeps track of your water usage for you.

The Hydro-Logic Flowmaster beeps when you hit the usage limit you have set to remind you to change filters.

Works by tracking the gallons of use.