Conductivity shows how readily water carries electrical current through dissolved ions; resistivity is its reciprocal; pH describes acidity or alkalinity. Read all three at the same reference temperature. None of these measurements alone proves that water is safe or identifies a specific contaminant.
Updated 20 July 2026: this guide covers unit conversion, calibration practice, alarm interpretation, the ISO 7888 reference, and buyer questions.
PT Watermart Perkasa supplies water-treatment instruments and components in Indonesia. In RO, demineralization, process-water, and drinking-water systems, conductivity, resistivity, and pH are most useful as trends against an accepted baseline—not as stand-alone quality labels.
| Parameter | What it measures | Common unit | Operational decision it supports |
|---|---|---|---|
| Conductivity | The solution’s ability to carry current through dissolved ions | µS/cm or mS/cm | Detect source-water change, RO salt passage, ionic leakage, or process concentration |
| Resistivity | Electrical resistance of the solution; reciprocal of conductivity | MΩ·cm | Monitor deionized or mixed-bed water at very low conductivity |
| pH | Hydrogen-ion activity on a logarithmic scale | pH unit | Control corrosion, coagulation, disinfection, and process compatibility |
At the same temperature and in matching units, the practical relationship is resistivity (MΩ·cm) = 1 / conductivity (µS/cm). For example, 0.10 µS/cm equals 10 MΩ·cm. Do not convert readings recorded at different temperatures without compensation, because conductivity changes with temperature.
How to Read Conductivity, Resistivity, and pH Together
Start with accepted raw-water and product-water baselines, then compare all three parameters at the same sample point and reference temperature. The matrix below identifies the next check; it is not a laboratory diagnosis.
| Data pattern | What to investigate | Verification step |
|---|---|---|
| Conductivity rises while pH is stable | Higher feed ionic load, RO salt passage, or a blending leak | Repeat the sample, check temperature, compare feed and permeate, then review RO pressure and flow |
| Resistivity declines gradually | Resin exhaustion, loop contamination, or poorer upstream RO quality | Review bed-volume trend, rinse condition, and samples before and after the mixed bed |
| pH shifts while conductivity is nearly stable | Acid/base dosing, CO₂ uptake, or an electrode problem | Recalibrate the pH meter and use a closed sample when CO₂ matters |
| All readings change abruptly | Wrong sample point, cross-contamination, temperature change, or process upset | Hold the decision, verify the instruments with standards, and resample |
ISO 7888:1985, confirmed current by ISO in 2023, specifies an electrical-conductivity measurement method for surface water, process water, treatment plants, and wastewater. The standard supports conductivity as a monitoring quantity while requiring attention to measurement interference and analytical purpose.
What is Conductivity in Water Treatment?
Conductivity is an indicator of the relative amount of mobile ionic species in water. A low reading means low ionic conduction, but it does not prove the absence of microorganisms, neutral organic compounds, or particles; the end-use specification must still define the required tests.
In reverse osmosis (RO), compare feed and permeate conductivity under stable operating conditions to calculate salt rejection: rejection (%) = (1 − permeate conductivity / feed conductivity) × 100. The trend can flag performance change, but it must be read with pressure, temperature, recovery, and flow; low feed conductivity alone does not prove that a membrane is operating more effectively.
Why is Resistivity Important?
Resistivity matters in demineralized and deionized water because small changes in ionic content become easier to see in this range. A high value indicates high electrical resistance, but pharmaceutical or semiconductor grades must never be assigned from resistivity alone; each user needs its own specification and verification method.
Furthermore, periodic resistivity measurements can aid in the performance monitoring of water treatment systems, such as ion exchange resins or filtration membranes. A sudden increase in mineral contamination, characterized by a decrease in resistivity values, can be an early indicator of the need for maintenance or replacement of system components. This is important not only to maintain water quality but also to maintain operational cost efficiency and minimize production downtime.
pH and its Effect on Water Quality
pH control affects corrosion, coagulation, carbonate equilibrium, and disinfectant chemistry. Because pH is logarithmic and electrodes are sensitive to temperature, junction condition, and storage solution, operators should retain calibration results with the process reading—not only the number shown on the display.
In food and beverage production, pH can affect flavour, formulation stability, corrosion, and the effectiveness of specific process stages. A compliant pH does not prove microbiological safety; that requires separate methods and acceptance limits.
Temperature compensation, automatic calibration, and digital outputs can reduce handling error, but they do not replace verification standards, electrode maintenance, and retained calibration records.
Set the pH target from the process need or product-water specification. Alarms should have warning and action limits plus a verification procedure so that one abnormal reading does not immediately trigger chemical overdosing.
Conductivity, resistivity, and pH provide fast process signals, but they do not replace contaminant or microbiological analysis. They are most useful when instruments are calibrated, sample points are consistent, temperature is recorded, and each trend has a defined operator response.
Instrument Measurement and Commissioning Checklist
- Define the sample point, working range, reference temperature, and alarm limits from the design basis or an accepted baseline.
- Select a suitable cell constant and instrument range; do not use a high-range sensor for ultrapure water unless its resolution is demonstrated.
- Verify calibration standards are in date and bracket the working range; record lot number, temperature, as-found result, and as-left result.
- Rinse the probe and vessel with sample, exclude bubbles, wait for a stable reading, and record temperature with the result.
- For pH, use two- or three-point calibration across the process range and check slope/offset against the instrument manual.
- At commissioning, compare the online instrument with a portable reference on the same sample and test the alarm or 4–20 mA output.
- Trend feed, permeate, and product readings. Investigate process changes before moving setpoints or replacing media.
For online monitoring, review industrial conductivity monitors and flowmeters. If the trend suggests salt passage or purity loss, assess the reverse-osmosis application and mixed-bed polishing application together with the resin, membranes, valves, and sample points. Drinking-water suitability still requires verification against the applicable health parameters; Indonesia’s current environmental-health reference at this update is Ministry of Health Regulation No. 2 of 2023.
Data Needed to Select a Conductivity Analyzer
Do not request an analyzer with only one target µS/cm value. Sensor range, materials, temperature compensation, mounting, and control outputs must match the process conditions and the existing panel.
| Data to provide | Example information | Selection impact |
|---|---|---|
| Measurement point and purpose | RO feed, permeate, mixed-bed outlet, or product alarm | Defines range, cell constant, and detection limit |
| Minimum, normal, and maximum values | Historical readings with sample temperature | Avoids poor resolution or frequent over-range readings |
| Process conditions | Temperature, pressure, flow, pipe material, and chemical exposure | Defines wetted materials and mounting method |
| Panel requirements | 4–20 mA, relays, communications, power, and enclosure | Defines transmitter and control integration |
| Verification method | Calibration standards, reference meter, and check interval | Defines sensor access and the commissioning plan |
Include installation photos, connection size, distance to the panel, and a sample trend. Complete data allow PT Watermart Perkasa to distinguish a general process sensor from an instrument intended for very low-conductivity water.
Buyer Questions about Water Conductivity
Is conductivity the same as TDS?
No. Conductivity is a direct electrical measurement, while many instruments display TDS as an estimate calculated from conductivity with a conversion factor. That factor depends on the ionic mixture, so converted TDS should not be confused with TDS established by a laboratory method.
What is a good conductivity value for water?
There is no single value for every use. Set limits from the product-water specification, an accepted baseline, and the downstream process; drinking water, RO feed, RO permeate, boiler water, and deionized water require different ranges and verification parameters.
When should a conductivity sensor be calibrated?
Follow the manufacturer and quality-system interval, then verify it after cleaning, sensor replacement, unsuitable storage, or disagreement with a reference instrument. Use an in-date standard that matches the working range.
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