Hydrologic Stealth-RO150/300 RO MembraneStealth RO150 300 RO Membrane reduces 89%+ of PPM of total dissolved solids. There are two indications as to when to change your membranes. 1. Test the RO and source inlet water to see what percentage of the inlet waters PPM the 150 is filtering out (rejection %). You should see approximately 98% of the inlet PPMs being removed when the membrane is new. If the rejection % falls under an acceptable level (typically 90%) its time to change the membrane.