Osmotic pressure estimate
Estimate osmotic pressure from TDS and temperature for NaCl-dominated waters — the starting point for RO feed pressure and membrane selection.
Results
| Estimated osmotic pressure | 1,4 bar |
|---|---|
| In psi | 20,3 psi |
| Rule of thumb | ≈ 0.7 bar per 1 000 mg/L TDS (NaCl basis, 25 °C) |
Feed pressure must exceed osmotic pressure plus the membrane’s net driving pressure and system losses before any permeate flows — as recovery rises, the concentrate-side osmotic pressure climbs well above this feed-side figure. The estimate assumes NaCl-dominated water; waters high in divalent ions run somewhat lower. Use a full analysis and the membrane manufacturer’s projection software for design.
Formula used
π = 0.0007 × TDS × (T + 273.15) ÷ 298.15
- π
- osmotic pressure, bar
- TDS
- total dissolved solids, mg/L
- T
- water temperature, °C
Industry rule of thumb (≈0.7 bar per 1 000 mg/L NaCl at 25 °C) scaled by absolute temperature. Note where the 0.7 comes from: ideal van 't Hoff behaviour for dilute sodium chloride gives about 0.85 bar per 1 000 mg/L, and the working figure is lower because the osmotic coefficient falls below unity at real feed concentrations. It is calibrated for brackish feeds in roughly the 1 000 to 10 000 mg/L range and it drifts outside that — it understates dilute feeds and is not a seawater figure. Waters dominated by divalent ions (Ca, Mg, SO4) run 10–30% lower again. Treat the result as an order-of-magnitude check with an uncertainty of tens of per cent, not a design number: for design, use the membrane manufacturer's projection software with a full ionic analysis.
Equipment for this duty
Send the water analysis and duty point, and the branch confirms exact selections, pricing and lead times — usually within one working day.
Request a quoteResults are indicative engineering estimates based on standard formulas and typical assumptions. They are not a design service; confirm any design against a full water analysis and the relevant manufacturer’s data.