Conductivity, TDS and Salinity: One Measurement, Three Readings

One sensor, three displays: conductivity, TDS and salinity are the same underlying measurement wearing different conversion factors. Confusing them produces numbers that look precise and mean nothing. Here is how the chain actually works.

Conductivity Is the Measurement

The instrument measures how easily the water carries current between electrodes — conductivity, in µS/cm or mS/cm. Everything else reported is arithmetic on top of that. Two sensor designs dominate:

• Contacting (electrode) cells pass current through the water between two or four electrodes; the cell constant (geometry factor) sets the range. Four-electrode designs reject fouling and polarization far better than two-electrode ones.
• Toroidal (inductive) sensors couple through the water magnetically — no metal contact, immune to coating and corrosion, ideal for high-conductivity or fouling service, at the price of a larger sensor body.

The Conversions

TDS (mg/L) = conductivity × an empirical factor, typically 0.5–0.7 depending on the ionic mix. The "0.65 default" in many instruments is right for nobody in particular. If TDS matters to your process, determine the factor gravimetrically on your actual water — once.

Salinity (PSU or g/kg) = conductivity mapped through the practical salinity scale, valid for seawater-like matrices. Reporting "salinity" for brackish process water or wastewater is a category error; use conductivity or a locally determined TDS factor.

Temperature Compensation: The Biggest Error Source

Conductivity changes about 2% per °C — enormous compared to instrument accuracy. All reporting references 25 °C, so compensation quality dominates field accuracy. Verify that the temperature element is fast and in the same flow as the cell; a slow temperature sensor in fast-changing water produces beautiful, wrong trends. For high-purity water, standard linear compensation fails — use instruments with dedicated ultrapure algorithms (which model water's own ionization).

Cell Constants and Range Matching

Choose the cell constant to the duty: low constants (0.01–0.1/cm) for ultrapure and drinking water, 1.0/cm for general process, 10/cm or toroidal for brines and concentrated streams. Running a 1.0 cell in reverse-osmosis permeate or a 0.01 cell in brine guarantees poor resolution where you need it most.

Practical Checklist

• Verify the temperature reading against a reference thermometer — quarterly.
• Wet-calibrate with a standard whose value sits inside your working range, not at the cell's theoretical center.
• Know which conversion your display shows and who set the factor.
• In fouling service, choose four-electrode or toroidal designs and add cleaning cycles.

Get the raw conductivity right, understand the conversion on top of it, and all three numbers become trustworthy.

Temperature Measurement Placement

Because compensation divides by a temperature-dependent factor, a temperature sensor that lags the conductivity cell by even 30 seconds creates phantom conductivity swings in batch processes and CIP cycles. In installations with fast temperature changes, choose sensors with the temperature element embedded at the measurement face, not behind the body. Verify compensation by measuring a standard at two temperatures — the readings should agree within the instrument's accuracy class.

Fouling Behavior by Sensor Type

Contacting cells lose sensitivity as scale or biofilm insulates the electrodes — readings drift low, slowly, plausibly. Four-electrode designs compensate by separating current drive from voltage sense. Toroidal sensors shrug off coatings entirely but can be fooled by deposits that narrow the bore. In lime-dosing or high-hardness service, plan acid-cleaning access for contacting cells or specify toroidal from the start.

Applications Across the Plant

• RO and ion exchange: permeate and product quality — low-range, high-accuracy cells with ultrapure temperature algorithms.
• CIP and chemical dosing: concentration control of acids and caustics — toroidal sensors in corrosion-resistant bodies.
• Wastewater and source water: salinity intrusion tracking and industrial-discharge detection — robust mid-range cells, fouling-tolerant.
• Cooling towers: cycles-of-concentration control — the blowdown loop lives on conductivity.

Handled with an honest conversion factor and a healthy respect for temperature compensation, conductivity is the cheapest reliable window into water chemistry available — three useful readings from one robust measurement.