RO water quality comes down to three numbers most people have never looked at: TDS, pH, and conductivity. Understanding what they mean — and what acceptable ranges look like — tells you whether your reverse osmosis system is working, when to replace membranes, and whether your water is suited to its intended use.

TDS: The Primary RO Performance Indicator

Total Dissolved Solids (TDS) measures the concentration of all dissolved inorganic and organic substances in water, expressed in parts per million (ppm) or milligrams per liter (mg/L). These include calcium, magnesium, sodium, chloride, sulfate, nitrate, bicarbonate, and hundreds of trace compounds.

TDS Ranges and What They Mean

What TDS Tells You About Your RO Membrane

A healthy RO membrane rejects 96-99% of incoming TDS. To calculate your system’s rejection rate:

Rejection % = ((Feed TDS – Permeate TDS) / Feed TDS) x 100

Example: Feed at 400 ppm, permeate at 12 ppm = 97% rejection. Good.
Same feed, permeate at 80 ppm = 80% rejection. Membrane fouling or damage — investigate immediately.

A drop in rejection rate is usually the first sign of membrane failure, before any visible symptoms. Check TDS monthly with an inexpensive pen meter ($15-30). When rejection drops below 90%, it’s time to replace membranes.

pH: What RO Does to Water Acidity

Pure water has a pH of 7.0 (neutral). Reverse osmosis tends to produce slightly acidic water — typically pH 5.5 to 6.8 — because the membrane removes alkalinity (bicarbonates) along with other minerals. The carbonic acid naturally dissolved in water then dominates, pulling pH down.

pH Ranges for Common RO Applications

For drinking water, a remineralization stage (calcite filter or mineral cartridge) raises pH to 7.0-7.5 and adds back beneficial calcium and magnesium. AMPAC residential and commercial systems include optional remineralization as a fourth-stage add-on.

Conductivity: The Engineer’s TDS

Electrical conductivity (EC) measures how well water conducts electricity, expressed in microsiemens per centimeter (uS/cm) or millisiemens per centimeter (mS/cm). Dissolved ions carry charge — so higher TDS = higher conductivity. The relationship is approximately:

TDS (ppm) = EC (uS/cm) x 0.5 to 0.7 (depending on ion composition)

Industrial water treatment systems often monitor conductivity in real time because conductivity sensors are continuous — unlike grab-sample TDS meters. AMPAC commercial RO systems can integrate inline conductivity sensors with 4-20mA outputs for SCADA or BMS integration.

Conductivity Targets by Application

When to Test and What to Do

Monthly TDS checks with a pen meter are sufficient for most residential systems. Commercial systems warrant weekly checks and a log — gradual TDS creep is easier to catch in trend data than in single readings.

When to act:

• TDS rejection below 90%: Replace membranes. Also check pre-filters — a clogged sediment filter increases pressure drop and stresses the membrane.
• pH below 5.5: Add remineralization or a calcite post-filter. Acidic water can corrode copper plumbing over time.
• TDS rising but rejection rate stable: Feed water quality has changed. Get a full water test to identify new contaminants.
• Sudden TDS spike: Check for membrane rupture, o-ring failure, or bypass valve position.

Get Your System Sized Right

The right RO system for your application depends on feed water TDS, required output TDS, flow rate, and temperature. Use AMPAC’s RO sizing calculator to get a system recommendation based on your parameters. For complex applications or high-TDS source water, our engineers provide free system design consultations.