Yes correct... 0 tds is only achieved after the DI stage.... Calibration I'm referring to is initial set-up... 3-1 or 4-1 ratio. If not set correctly you won't get 0 tds.The OP stated "RO unit" which will never have 0 TDS since RO is only 90-98% efficient. RO/DI can and should have 0 TDS and they did state they have 0 TDS from their RO/DI after a bit which is normal once TDS creep clears. Russo I think you mean calibrate their TDS meter but again from the RO only portion of the system you will never have 0 TDS, only from the final RO/DI.