The short answer
Conductivity
Conductivity is the ease with which water carries current, which rises with the dissolved ions in it, so it is the quickest indicator of how much salt a water carries without saying which salt. It is measured with a contacting sensor, two or four electrodes with a known geometry expressed as a cell constant, or with an inductive sensor that induces a current in a loop of the liquid and reads it, which has no electrodes to foul or polarize and suits dirty and high-conductivity water. The reading depends strongly on temperature, so every sensor carries a temperature element and the transmitter compensates to a reference of 25 degrees Celsius. In water systems it monitors membrane feed and permeate to compute rejection, watches brine and concentrate, tracks blending of sources, controls blowdown, and flags a change in a source or an intrusion in a distribution system; in each case the number is meaningful relative to a baseline and the temperature compensation has to be right.
Key points
- Conductivity indicates total dissolved ions, not which ions; it is a ratio measurement against a baseline.
- Contacting sensors for clean water and low range; inductive sensors for dirty, fouling, or high-conductivity water.
- The cell constant sets the range; match it to the water, and use four-electrode or inductive cells for high ranges.
- Temperature compensation to 25 degrees Celsius is essential and the coefficient depends on the water.
- Calibrate with standard solutions of known conductivity, and keep the sensor clean and free of bubbles.
Sensors
| Sensor | Principle | Range | Where |
|---|---|---|---|
| Two-electrode contacting | Current between two electrodes of known geometry | Low to moderate; polarizes at high conductivity | Clean water: permeate, boiler and distilled water, low-range monitoring |
| Four-electrode contacting | Current through outer electrodes, voltage sensed at inner ones, which removes polarization | Wide, into the high range | Feed water, blending, general plant use |
| Inductive or toroidal | A drive coil induces current in a loop of liquid through the sensor bore; a sense coil reads it | Moderate to very high | Brine, concentrate, wastewater, fouling and corrosive liquids |
Cell constant and range
A contacting cell has a constant, in reciprocal centimeters, that relates the measured conductance to the conductivity of the water: a constant of 0.01 suits very pure water, 0.1 suits permeate and low-range water, 1.0 suits most raw and treated water, and 10 suits brine. A cell with the wrong constant for the water reads at the edge of its range with poor resolution or polarizes. The constant is on the sensor label and is entered in the transmitter, and a sensor replaced with one of a different constant reads wrong until the transmitter is updated.
Temperature
Conductivity rises with temperature by roughly two percent per degree Celsius for most natural waters, and the transmitter compensates the reading to 25 degrees Celsius using the temperature element in the sensor and a coefficient. The coefficient is not the same for every water: pure water follows a nonlinear curve, brines and acids have their own values, and a transmitter set to the wrong coefficient reads a temperature change as a conductivity change. The setting is chosen from the water type and checked by comparing readings on the same sample at two temperatures. A failed temperature element is the commonest reason a conductivity reading drifts with the weather.
κ_25 = κ_T ÷ (1 + α × (T − 25))
- κ_25 = conductivity compensated to 25 °C
- κ_T = measured conductivity at temperature T
- α = temperature coefficient, about 0.02 per °C for natural waters
Total dissolved solids
Total dissolved solids is a mass concentration measured by evaporation in the laboratory; conductivity is an electrical property. The two are related for a given water by a factor, usually between 0.5 and 0.7 milligrams per liter per microsiemens per centimeter, that depends on which ions are present. A transmitter that displays total dissolved solids is multiplying conductivity by a factor someone entered, and the factor is checked against laboratory results for that water. Reporting conductivity and stating the factor is more honest than reporting a derived number as if it were measured.
Uses
- Membrane monitoring
- Feed and permeate conductivity give the salt rejection of a reverse osmosis or nanofiltration train; a rising permeate conductivity is a damaged membrane, a failed seal, or a fouled element. Concentrate conductivity tracks recovery.
- Blending
- The conductivity of a blended water follows the proportions of the sources; a controller can hold a blend to a conductivity target.
- Blowdown and concentrate control
- Cooling towers, boilers, and concentrate systems are controlled on conductivity to hold cycles of concentration.
- Distribution monitoring
- A step change in conductivity at a monitoring point is a change of source, a cross-connection, or an intrusion, and it arrives before any other instrument notices.
- Wastewater
- Conductivity tracks industrial discharges, saltwater infiltration, and the strength of sidestreams.
Rejection (%) = (1 − κ_permeate ÷ κ_feed) × 100
- κ_permeate = permeate conductivity, compensated
- κ_feed = feed conductivity, compensated; some methods use the average of feed and concentrate
Calibration and problems
- Calibrate with a standard solution near the operating range, at a known temperature, with the sensor fully immersed and free of bubbles; common standards are near 1413 and 12880 microsiemens per centimeter.
- Clean contacting electrodes with the manufacturer method; a fouled electrode reads low, and a scaled one reads low and slow.
- Air bubbles trapped in a contacting cell or an inductive bore read as low conductivity; mount the sensor so that bubbles escape.
- Polarization on a two-electrode cell at high conductivity reads low; use a four-electrode or inductive sensor.
- Inductive sensors need clearance from pipe walls; a sensor too close to a metal wall reads wrong, and the installation factor is set for the mounting.
- Verify the temperature element against a thermometer; most drift is temperature compensation.
Frequently asked questions
- Why does the conductivity reading change with the weather?
- The temperature compensation is wrong or the temperature element has failed, so the seasonal temperature change appears as a conductivity change. Verify the temperature reading and the coefficient.
- Which sensor for a reverse osmosis permeate?
- A contacting sensor with a low cell constant, 0.1 or so, since permeate conductivity is low and clean. The feed and concentrate use higher constants or an inductive sensor.
- Can conductivity tell me the chloride level?
- Only by a correlation for that water, established against laboratory chloride results, and only if the other ions stay in proportion. It is a proxy, not a chloride measurement.
- The two conductivity meters on the same pipe disagree by 5 percent.
- Different cell constants entered, different temperature coefficients, or one sensor fouled. Check the constant and coefficient settings first, then calibrate both in the same standard.
Related topics
- pH MeasurementGlass electrodes, reference junctions, and why pH is the measurement that drifts: how the sensor works, temperature compensation, two-point buffer calibration, mounting rules, the failure modes of the reference, and using the signal for chemical feed control.
- Turbidity MeasurementNephelometric turbidimeters for filter effluent and raw water: how 90-degree scattering is measured, the methods and the NTU, low-range versus high-range instruments, bubbles and the debubbler, formazin calibration and verification standards, and the compliance record the instrument exists to produce.
- Calibration ProceduresHow a utility calibrates its instruments consistently: the as-found and as-left discipline, tolerance from the loop rather than the data sheet, the reference standard and its traceability, the five-point check with hysteresis, adjusting only when needed, intervals set from history, and the procedure written once per instrument type.
- Calibration TroubleshootingWhen an instrument will not calibrate or stay calibrated: wrong or expired standards, slope and offset out of range, compensation errors, procedure mistakes, loop scaling that disagrees with the instrument, and deciding when to adjust, replace.
- 4-20 mA Signals from the Instrument SideHow a 4-20 mA current loop is powered and wired at the transmitter, why the loop resistance budget matters, and how to check a loop with a meter.
- Signal ValidationCatching a failed analog input in logic before it runs a pump on a dead transmitter: range checks, module status bits, frozen-value detection, rate checks, and what the program does with a bad value.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.