Note: This reverse osmosis unit has a calcite/GAC cartridge. For that reason, the standard method for testing TDS to verify membrane performance has to be altered.
The calcite cartridge, which increases pH, also adds calcium carbonate and hence TDS (total dissolved solids) to the water that comes from the faucet. You will not get an accurate TDS reading from the faucet.
Turn off the inlet water to the unit.
Close the valve at the top of the storage tank.
Remove the tube that connects the tank to the tee at the top of the RO unit from the tee.
Using a glass to catch the water, slowly open the tank valve and run water into the glass. Run one glassful to assure a fresh sample, then fill the glass again and test the water.
(whole house & well units)