RO Water Check
Accurate Sampling

TDS Creep & Remineralization: Avoid Condemning a Good Membrane

Two harmless phenomena cause healthy RO systems to show high TDS numbers on a handheld meter: overnight TDS creep across a resting membrane and intentional mineral addition from a remineralization post-filter.

1. What Is TDS Creep (and Why Tankless Systems Show It More)?

When your RO system sits idle overnight, pressurized feed water on one side of the semi-permeable membrane slowly equalizes tiny amounts of dissolved ions into the stagnant purified water sitting inside the membrane housing. The first 200–500 mL drawn in the morning can read 2× to 5× higher than steady-state production water.

In a tank-based RO, that first high-TDS slug is diluted into 2–3 gallons of stored water. In a tankless RO, water flows straight from the membrane to your glass, so the first 10–20 seconds can read dramatically higher until fresh permeate flushes the housing (which is why many tankless units run an automatic membrane flush cycle).

0 sec (First cup) 30 sec flush 60+ sec (Steady state) High Low Illustrative drop as standing permeate clears
Figure 4 — Illustrative TDS Creep Washout Curve: Schematic trend (not a live lab measurement). Always flush the RO faucet for 30–60 seconds and rinse your testing cup with RO water before recording a membrane health reading.

2. How Different Post-Stages Affect Your TDS Meter

Post-Membrane Stage Typical TDS Impact Why It Happens
Post-Carbon Polishing Filter 0 to +5 ppm (after initial flush) Removes residual tastes/odors; only new un-flushed cartridges shed temporary carbon fines.
Calcite Remineralization Cartridge +15 to +50 ppm Dissolves calcium carbonate to buffer acidic RO water and raise pH toward neutral.
Calcite + Corosex Alkaline Blend +30 to +80+ ppm Adds magnesium oxide alongside calcium, pushing pH higher (often 8.0–9.0) and raising conductivity.
UV Sterilizer Stage 0 ppm Ultraviolet light neutralizes microorganisms without adding or removing dissolved ions.

3. How to Sample Pure Membrane Water (Point B)

  1. Shut off the storage tank ball valve (if equipped) and cold supply valve briefly to relieve line pressure.
  2. Locate the 1/4-inch tubing exiting the RO membrane permeate port (before it enters the remineralization cartridge).
  3. Push in the collet ring on the quick-connect fitting to release the tubing, aim it into a clean glass, and turn the cold supply back on for 30–60 seconds before collecting your sample.
  4. Re-seat the tubing firmly until it stops, tug gently to verify the lock, and test your reading in our RO TDS Rejection Calculator.