Start making RO or RODI water (open a pure water line).  All systems have a RO ONLY line, our reefkeeper systems have a RODI line or DI bypass. If you have a drinking water system, we advise you close the blue tank ball valve on the pressure tank and just turn on the RO faucet for this step.  If you have a RODI unit and are lucky enough to have a DI bypass, then you will open the blue ball valve as to open the DI bypass so we don't waste any DI resin (If you do not have a DI bypass on your RODI system, you could still make RODI water or disconnect the tubing going INTO the final DI filter). 2. Fast flush your system for 15 minutes. On ALL of our systems, this simply means to turn the black handle on the flow restrictor to the right 90 degrees so the black handle is pointed TOWARDS the arrow on the flow restrictor (PARALLEL).  HINT> there is only ONE black handle on our units and it can be found easily by looking down from above your system.   3. Return the black handle on the flow restrictor back to normal operations mode (closed).  Just turn the black handle to the left 90 degrees so it is pointed perpendicular to the tubing and pointed AWAY from the drain lines tubing. NOTE> This process removes the "tds creep" in the lines that do not accurately represent the water quality of the system. 4. Make ONE gallon of RO water. (about 10-15 minutes) .  NOTE> This will remove the remaining "tds creep" in your system that we do not want to pick up in our TDS test.   5. Get a clean cup and your TDS meter.  Do not use the cap of your TDS meter.  If you need a TDS meter [click here] ! 6. Fill your cup with RO water and take a TDS reading of the RO water sample.  NOTE> Stir the tds meter in the cup with the sample so there are no air bubbles around the probes of the tds meter.  Allow 30 seconds to pass before you take note of the number appearing on your TDS meter.  As a side note, if you have water colder than 77 degrees a non temperature adjusted tds meter will show LESS tds ppm than what is actually present. The opposite is true aswell...if your water sample is over 77 degrees, your tds meter's ppm will overstate how much tds is in your sample. (one can adjust about 2% per degree celsius for the correct tds present in a water sample). 7. Repeat step 6 and take a tds reading of your TAP (unflitered) water.  NOTE> Before doing this, empty your cup with the RO water sample and shake out the excess water in the cup and on the TDS meter.  Then go to your sink faucet (or somewhere close) and take your sample. 8. Compare RO tds reading to TAP water tds reading.  Your RO water tds reading should be approximately 5% of the TAP water tds reading. If your TDS reading is more than 10% of your tap water TDS, it's time to replace your membrane [click here for membranes]. EXAMPLE: Your RO tds reading is 10 ppm tds.  Your TAP water tds reading is 200 ppm tds.  This is a 95% rejection rate. [95%=100%-(10ppm/200ppm)]. 9. If you have a RODI unit you have a DI resin stage. Now is a great time to take a TDS reading of the RODI water that is leaving the unit. It should be close to ZERO! If this is a new DI filter and your meter reads higher than zero, your TDS meter is WRONG! FYI: A quick and easy test to determine if your DI is exhausting too quickly is calculated as follows: 3,000 divided by the TDS/PPM of your RO water = Total estimated gallons of ZERO TDS water the resin will process before it starts exhausting. RODI EXAMPLE: If your tap water TDS is 200 PPM ....  the TDS of your RO water (after the membrane) will have about 5% remaining, so 10 PPM. IF you have a standard 10" refillable DI cartridge, divide 3000 by this 10 PPM and that equals about 300 gallons of RODI water you will make before your DI resin is exhausted. If you have a in-Line DI filter will last only half as long. The 20" refillables will last twice as long. If you do not fast flush your system before you start making water, TDS creep can reduce this volume.