{"id":89027,"date":"2026-07-08T09:00:00","date_gmt":"2026-07-08T09:00:00","guid":{"rendered":"https:\/\/www.ampac1.com\/blog\/?p=89027"},"modified":"2026-06-30T01:16:19","modified_gmt":"2026-06-30T01:16:19","slug":"ro-water-quality-tds-ph-conductivity-guide","status":"publish","type":"post","link":"https:\/\/www.ampac1.com\/blog\/ro-water-quality-tds-ph-conductivity-guide\/","title":{"rendered":"RO Water Quality: What TDS, pH, and Conductivity Actually Mean for Your System"},"content":{"rendered":"
\nQuick Answer:<\/strong> RO water quality is measured by four parameters: TDS (total dissolved solids, target <50 ppm for most commercial use), conductivity (proportional to TDS, target <80 \u00b5S\/cm), pH (typically 5.5\u20136.5 post-RO due to dissolved CO2, acceptable for most uses), and hardness (should be near zero after a properly functioning membrane). A salt rejection rate below 95% on a system rated for 98%+ is the clearest signal that the membrane needs attention \u2014 either cleaning, re-evaluation of pretreatment, or replacement.\n<\/div>\n

Most RO operators check a TDS pen once a week, see a number under 50, and call it good. That’s not wrong \u2014 but it’s incomplete. TDS tells you the membrane is rejecting salts. It doesn’t tell you which<\/em> salts, whether the reading is trending in the wrong direction, or whether your pretreatment is quietly degrading the membrane one week at a time. The four parameters below together give you an actual picture of system health.<\/p>\n

The Four Core RO Water Quality Parameters<\/h2>\n

1. TDS \u2014 Total Dissolved Solids<\/h3>\n

TDS is the aggregate weight of all dissolved minerals, salts, and organics in your water, measured in parts per million (ppm) or milligrams per liter (mg\/L). For RO permeate (the product water), TDS is the primary go\/no-go metric.<\/p>\n\n\n\n\n\n\n\n\n\n\n\n
Application<\/th>\nTarget Permeate TDS<\/th>\nReject if Above<\/th>\n<\/tr>\n<\/thead>\n
Restaurant \/ food service drinking water<\/td>\n<50 ppm<\/td>\n150 ppm<\/td>\n<\/tr>\n
Water store \/ vending<\/td>\n<30 ppm<\/td>\n50 ppm<\/td>\n<\/tr>\n
Boiler feedwater (low pressure, <300 psi)<\/td>\n<100 ppm<\/td>\n150 ppm<\/td>\n<\/tr>\n
Boiler feedwater (high pressure, >600 psi)<\/td>\n<1 ppm<\/td>\n5 ppm (needs EDI)<\/td>\n<\/tr>\n
Pharmaceutical (USP purified water)<\/td>\n<0.5 ppm (500 \u00b5g\/L)<\/td>\nPer USP <645><\/td>\n<\/tr>\n
Laboratory (Type II water)<\/td>\n<1 ppm<\/td>\n5 ppm<\/td>\n<\/tr>\n
Semiconductor \/ Type I UPW<\/td>\n<0.005 ppm (5 ppb)<\/td>\nResistivity <18 M\u03a9\u00b7cm<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

What a rising TDS tells you:<\/strong> If your permeate TDS increases by 10\u201315% from the system’s baseline, that’s an early warning. Gradual TDS rise over weeks usually means membrane fouling or scaling. A sudden jump \u2014 30%+ overnight \u2014 suggests a mechanical failure: broken O-ring, telescoped membrane element, or a bypass leak.<\/p>\n

2. Conductivity<\/h3>\n

Conductivity measures how well water carries an electrical charge \u2014 which is directly proportional to its dissolved ion content. It’s essentially TDS measured electronically, in microsiemens per centimeter (\u00b5S\/cm). Most inline RO monitors measure conductivity rather than TDS and convert using a factor (typically 0.5\u20130.7 depending on ion composition).<\/p>\n

\nWhy conductivity matters more than TDS for trending:<\/strong> Conductivity sensors can be installed inline and read continuously. TDS pens require grab samples. For industrial systems, continuous conductivity monitoring across the membrane (feed vs. permeate) gives you real-time salt rejection calculation without pulling water samples.\n<\/div>\n

Salt rejection formula: Rejection (%) = (1 - Permeate Conductivity \/ Feed Conductivity) \u00d7 100<\/code><\/p>\n

A new membrane should reject 97\u201399% of ions. If rejection drops to 90% or below on a system that was previously performing at 98%, the membrane is either fouled, scaled, or damaged.<\/p>\n

3. pH<\/h3>\n

RO membranes don’t remove dissolved CO2 \u2014 only ions. Carbon dioxide passes through freely, then dissolves into the permeate as carbonic acid, dropping pH to 5.5\u20136.5 even when the feed water is neutral. This is normal. It confuses a lot of operators who test permeate pH, see 5.8, and assume something is wrong.<\/p>\n\n\n\n\n\n\n\n
pH Range (Permeate)<\/th>\nLikely Cause<\/th>\nAction<\/th>\n<\/tr>\n<\/thead>\n
5.5 \u2013 6.5<\/td>\nNormal \u2014 CO2 passthrough<\/td>\nNone required for most uses<\/td>\n<\/tr>\n
<5.5<\/td>\nHigh CO2 in feed, acidic source water, or acid injection issue<\/td>\nCheck feed pH; test for acid dosing fault<\/td>\n<\/tr>\n
>7.5<\/td>\nCaustic contamination, NaOH carryover from CIP, or membrane degradation<\/td>\nInvestigate pretreatment chemistry immediately<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

pH becomes critical in two contexts: boiler feedwater (where low pH accelerates corrosion) and pharmaceutical water (where USP standards define conductivity limits at measured pH). For drinking water applications, if end users are concerned about acidity, a calcite or alkaline post-filter resolves it without affecting membrane performance.<\/p>\n

4. Hardness<\/h3>\n

Hardness (calcium and magnesium ions) is what the RO membrane was primarily designed to reject. Permeate hardness should be near zero \u2014 typically <1 grain per gallon (GPG) or <17 ppm as CaCO3. If hardness is measurable in the permeate, one of three things is happening:<\/p>\n