RO/DI THREE-POINT TDS MEASUREMENT SHEET
Guide: https://zoolory.com/aquarium/rodi-water-high-tds/

F = feed water actually entering the RO/DI unit
R = RO permeate before DI, not the waste/reject line
D = output after the last DI stage, before storage

Date / system / membrane model and GPD: ______________________
Last sediment / carbon / DI change: _________________________
Idle period / feed temperature: _____________________________
Meter / ppm conversion setting / calibration check: _________
Running pressure / gauge location: __________________________
Startup or flush procedure / duration: ______________________
Normal production valve position restored: __________________

All three readings must use ppm and the same conversion setting.
Check meter seating, calibration and sampling containers first.
Finish the specified startup procedure and discard product water
until stable. Record pressure during normal production.

Readings in ppm        Time since restart   F    R    D
Startup                _________________   ___  ___  ___
Stable set 1           _________________   ___  ___  ___
Stable set 2           _________________   ___  ___  ___
After service          _________________   ___  ___  ___

RO rejection = (1 - R / F) x 100 = ______________ %
Example only: F = 250 ppm, R = 10 ppm gives 96% rejection.
Do not calculate with zero, missing or unreliable feed TDS.
If R exceeds F, repeat sampling and check lines, units and meter.

Manual's expected rejection and test conditions: _____________
Manual's DI replacement or exhaustion criteria: ______________
Stage to check / action taken: _______________________________
Pressure / temperature after service: ________________________
Post-service result / next action: ___________________________

Compare rejection with the installed membrane's documentation
and previous measurements under similar operating conditions.
96% is a worked example, not a universal replacement threshold.
If rejection is appropriate but D rises, inspect the DI stage.
If rejection is poor, check pressure and pretreatment before
replacing the membrane. Follow the specific resin instructions.
Zero displayed TDS does not prove sterility or aquarium readiness.