The SM-1 is an easy way for you to have access to a single TDS measurement anytime. If you know your inlet TDS and it's always the same, then you really only need to know the outlet measurement. The SM-1 will give that to you instantly and constantly. Simply turn the unit on and seconds later the large LCD will show you your TDS level. Stop guessing when your RO membrane needs to be replaced and know it the scientific way.