A higher permeate-conductivity reading is a reason to investigate, not a complete diagnosis or an automatic instruction to replace an element. Before discussing membrane performance, buyer and supplier should agree what was measured, where it was measured and which reference applies. This worksheet organizes a technical enquiry; it does not establish a universal failure threshold or a field acceptance guarantee.
1. Identify the metric and the sampling boundary
Label each result as laboratory concentration, conductivity or a meter's calculated TDS value. Record the unit, analytical method or meter setting, sample identity, location and time. USGS explains conductivity through dissolved ions; it is a different measurement from a mass concentration. Ask which conversion, if any, the meter applies instead of assuming that values in µS/cm and mg/L can be exchanged without an agreed basis.
Mark the proposed feed and permeate sampling points on the actual piping diagram. Ask whether the permeate point is before storage, blending, remineralization or other dosing, and whether it represents one vessel, one stage or the combined plant. A raw-water sample taken upstream of pretreatment and a distribution-tank sample taken later do not automatically describe the same membrane boundary. Obtain a matched sampling plan before comparing their ratio.
2. Calculate a defined ratio without overstating its meaning
For paired concentration results on the same defined constituent and analytical basis, write the calculation explicitly: apparent passage = 100 × Cp/Cf; apparent rejection = 100 × (1 − Cp/Cf), where Cf and Cp are feed and permeate concentrations in the same units. As hypothetical arithmetic only, Cf = 1000 mg/L and Cp = 5 mg/L give 0.5% passage and 99.5% rejection. This is not a BW-4040 test result, an engineering projection or a normalized performance value.
A conductivity-based rejection indicator can be useful when the agreed test method defines it; label the result accordingly. Do not substitute a concentration in one part of the ratio and conductivity in the other. Retain original measurements alongside calculated fields, including detection limits and results reported below a method's quantification limit. Ask the reviewer how uncertainty affects the decision rather than reporting extra decimal places as evidence of accuracy.
3. Compare with the right reference and assign the next check
BOKAWATER BW-4040 is listed at 2500 GPD (9.5 m³/d) and 99.60% stabilized salt rejection with 2000 mg/L NaCl, 225 psi (1.55 MPa), 25 °C, pH 7.5–8.0 and 15% recovery. Keep that complete catalog test package with the model. A site reading under other conditions is not a direct repeat of that test. DuPont identifies feed conditions as performance influences and discusses normalization; use the method approved for the actual elements and plant.
Send synchronized flows, pressures, temperature, pH, recovery, instrument checks and the relevant water analysis with the sample results. Include the installed model, vessel-position map, clean baseline, recent dosing changes, shutdowns, cleaning and maintenance events. Ask the supplier to separate measurement validation, operating-condition review and any further integrity investigation, recording the evidence and responsible person for each step. Do not infer a leak location, cleaning recipe, warranty decision or replacement compatibility from one conductivity trend.
Data to collect before comparing proposals
| Parameter / interface | Unit / format | Why it matters |
|---|---|---|
| Review item | Evidence to retain | Decision to close |
| Metric | Concentration, conductivity or calculated TDS | Is the basis defined? |
| Sampling boundary | Paired locations, times and operating state | Do the samples represent one boundary? |
| Instrumentation | Units, settings and verification record | Are readings comparable? |
| Calculation | Original Cf, Cp and stated formula | Is it an apparent or normalized value? |
| Model reference | Model and full catalog test conditions | Is this the relevant reference? |
| Operating point | Flow, pressure, temperature and recovery | Which conditions changed? |
| History | Dosing, cleaning, shutdown and position map | What evidence supports investigation? |
| Next action | Named reviewer and approved check | What remains unconfirmed? |
Technical references
- BOKAWATER — BW-4040 brackish-water RO membrane, item BK-BW-4040 ↗Source checked:
Evidence scope — original English record
Confirms the existing model and listed flow, stabilized rejection and complete test-condition package. These are catalog values, not independently verified field performance or a replacement approval. No additional operating limit is adopted.
- DuPont — Reverse Osmosis, influencing factors ↗Source checked:
Evidence scope — original English record
General background on feed temperature, pH, salt concentration and normalization when reviewing membrane performance. No DuPont product rating, numerical cleaning trigger or software compatibility claim is transferred to BOKAWATER.
- USGS — Conductivity (Electrical Conductance) and Water ↗Source checked:
Evidence scope — original English record
Explains that dissolved ions enable water to conduct electricity. This supports identifying the measurement type; it does not supply a universal conductivity-to-concentration conversion or a membrane acceptance procedure. The worksheet and hypothetical arithmetic are original review aids.